Make IRWitnessTable HOISTABLE (#6417)

# Make `IRWitnessTable` Hoistable

## Intention of the PR
This commit makes `IRWitnessTable` Hoistable so that we can avoid duplicated `IRWitnessTable`.


## Problems
This commit tries to address the following issues arise after turning `IRWitnessTable` into Hoistable:
 1. A Hoistable instance is immutable.
 2. When tries to create a duplicated child, you will get a previously created instance of `IRWitnessTable`, instead of a new one.
 3. We don't actually want to hoist `IRWitnessTable`.
 4. There can be only one instance of Hoistable and it cannot appear as childs multiple times.
 5. Different import/export mangled names were used for the same Witness-table when its type is "enum" interface.


## Implementation

### Solution for "1. A Hoistable instance is immutable."
`IRWitnessTable::setConcreteType()` is removed, because when an `IRInst` is Hoistable, it is treated as immutable. Any `IRInst::setXXX()` methods don't work anymore.

There were two places calling `setConcreteType()` and their logic had to change little bit.

`DeclLoweringVisitor::visitInheritanceDecl()` in `source/slang/slang-lower-to-ir.cpp` was calling `setConcreteType()`. It had a little strange logic around `lowerType()`. The `IRWitnessTable` was added with `context->setGlobalValue()` first and its `concreteType` was changed later. This commit works around in a way that it sets the parent of `IRWitnessTable` temporarily and reset it with the correct `IRWitnessTable`. Without this logic, it went into an infinite recursion.

`AutoDiffPass::fillDifferentialTypeImplementation()` in `source/slang/slang-ir-autodiff.cpp` was calling `setConcreteType()`. It was changing the concreteType of `innerResult.diffWitness`. This commit creates a new `IRWitnessTable` and copies its `IRWitnessTableEntry`.


### Solution for "2. When tries to create a duplicated child, you will get a previously created instance of IRWitnessTable, instead of a new one"

After a call to `IRBuilder::createWitnessTable()`, this commit checks if the returned `IRWitnessTable` is a brand new or not. If it is not a new one, we have to avoid adding the decorations and children.

This commit decides when to add decorations and children based on whether `IRWitnessTable` has any of decorations or children already. It doesn't seem like a proper way to check. But when I tried, it was difficult to find a bottleneck point where the decorations and children are added to `IRWitnessTable` first time. Note that we are not trying to find when `IRWitnessTable` is created for the first time; we need to find if the decorations and children were added once.

It might be fine to have duplicated `IRWitnessTableEntry` in most of the cases, but I noticed that it fails an assertion check when `shouldDeepCloneWitnessTable()` returns false in `cloneWitnessTableImpl()`.


### Solution for "3. We don't actually want to hoist IRWitnessTable."

The reason why this commit makes `IRWitnessTable` is to prevent the duplicated instances of `IRInst`. But we don't really want to "Hoist" them.

When an `IRWitnessTable` gets Hoisted out, it causes unexpected problems and the specialization process fails due to the missing `IRWitnessTable` in the input.

This commit prevent from hoisting `IRWitnessTable` in `_replaceInstUsesWith()`. The way this is implemented feel little hack but we discussed on Slack and decided to go with this. One of the proper approaches could be to add a new flag in `IROpFlags` and have a new one like `kIROpFlag_Deduplicate`, which is different from just `kIROpFlag_Hoistable`.


### Solution for "4. There can be only one instance of Hoistable and it cannot appear as childs multiple times."

When `IRWitnessTable` is Hoistable, there can be only a unique set of instances. And we cannot have an instance as a duplicated childs. It is because `IRInst` has only one set of `IRInst* next` and `IRInst* prev`.

Before this commit, an instance of `IRGeneral` could have duplicated instances of `IRWitnessTable`. As an example, `IInteger` interface inherits two other interfaces, `IArithmetic` and `ILogical`. And they both inherits from `IComparable`.
```
interface IInteger : IArithmetic, ILogical {}
interface IArithmetic : IComparable {}
interface ILogical : IComparable
```
When we specialize it in `specializeGenericImpl()`, an `IRBlock` gets the following list of children:
- IRWitnessTable for IComparable,
- IRWitnessTable for IArithmetic,
- IRWitnessTable for IComparable,
- IRWitnessTable for ILogical,

For the cloning during the specialize, "IRWitnessTable for `IComparable`" must be cloned before the cloning of "IRWitnessTable for `IArithmetic`". Because "IRWitnessTable for `IArithmetic`" refers "IRWitnessTable for `IComparable`" as its `IRWitnessTableEntry`. The order they appear in the `IRBlock` as children decides which instances will be cloned first. And "IRWitnessTable for `IComparable`" must appear before "IRWitnessTable for `IArithmetic`".

Note that "IRWitnessTable for `IComparable`" appears twice, The first one was added for "IRWitnessTable for `IArithmetic`". And the second one is added for "IRWitnessTable for `ILogical`".

With this commit "IRWitnessTable for `IComparable`" can appear as a child only once in `IRBlock`. So it causes an error if it gets the following list:
- IRWitnessTable for IArithmetic,
- IRWitnessTable for IComparable,
- IRWitnessTable for ILogical,

In order to resolve the problem, "IRWitnessTable for `IComparable`" must appear before both "IRWitnessTable for `IArithmetic`" and "IRWitnessTable for `ILogical`" as following:
- IRWitnessTable for IComparable,
- IRWitnessTable for IArithmetic,
- IRWitnessTable for ILogical,

To address the problem, the instances of `IRWitnessTable` is always added to the end of the children list. If it is already added to the list, we don't move. This works out because the AST tree is built based on the dependencies.


### Solution for "5. Different import/export mangled names were used for the same Witness-table when its type is "enum" interface."

This issue was found while testing with Falcor tests where it uses Conformance-type feature of Slang.
We are using different import and export mangled names for a same Witness-table when the witness-table is for "Enum" interface.

The way we simplify the implementation of "Enum" causes a problem when it comes to generate export/import for the witness-table. And the exact repro step is still unclear.

There were two suggested solutions for the problem and this PR adopted the first option for now. Maybe we want to improve it with the second option later.

option 1, when we produce mangled names for those witness-table, we can use a mangled name with the underlying "int" type instead of the name of the enum type. In this way, all witness-tables for enum types whose underlying type is same will get the same mangled name. It will allow us to deduplicate the witness-table during the linking.

option 2, we can preserve type info for enum type when generating IR. We can still erase all other uses of the type info of enum types for now. But when we generate the witness-table, instead of filling the conforming type operand to IntType, we fill it as EnumType(IntType) where EnumType is a new global IROp code to represent all enum types (like InterfaceType/StructType). This way the operands for the two witness-tables will be different.

"option 1" is more quick and dirty and "option 2" is more proper way to address it.
I should go with "option 1" and improve it with "option 2" approach later.

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