Some Slang API additions (#1195)

* Some Slang API additions

These are additions to the public Slang API that came up while I was trying to write an example to demonstrate GPU printing. The additions aren't strictly necessary for the example, but I found these to be missing services when writing the code the way I wanted to.

The main public changes are:

* There is a new distinct `IEntryPoint` interface which inherit from `IComponentType` (much like `IModule` before). For now this doesn't expose any new functions on top of `IComponentType`, but I expect it to do so eventually.

* It is now possible to get the `IModule` for a specific translation unit in a compile request with `spCompileRequest_getModule`. Even for a compile request that had only one translation unit this is *not* the same object as gets returned by `spCompileRequest_getProgram`. The latter should probably be called `*_getLinkedProgram` because it returns a composite component type that links the module with everything it `import`s.

* An `IModule` can look up entry points declared in the module. Eventually a module should support looking up most of the declarations in the module (e.g., types) by name, but entry points are an obvious case.

* A new `link()` operation is added to `IComponentType`. It is possible to have component types that have unsatisfied dependencies, such that trying to generate kernel code from them will fail. The `link()` operation tries to produce a new composite component type that combines a component with its dependencies, to enable code generation. The implementation of end-to-end compilation was using a function like this internally, but it hadn't been exposed to the API.

Notes on the implementation:

* The list of entry points declared in a given translation unit has moved from `TranslationUnitRequest` to the `Module` inside of it.

* `EntryPoint` now has to do a song and dance much like `Module` to both inherit from `ComponentType` and support the `IEntryPoint` interface.

* The `Session* m_session` member in `Linkage` (in terms of public API, this is the `slang::ISession` holding a pointer to the `slang::IGlobalSession`) has been changed to a `RefPtr`. Without this change an application can't just hold onto a `ComPtr<slang::ISession>`; they also need to retain the `IGlobalSession` or things will crash. The new behavior seems more correct, but I worry that it might introduce a leak.

* The `asInternal` operation for `IComponentType` had to be updated to not just perform a cast. A type like `Module` has two `IComponentType` sub-objects, and only one of these is at the same address as the `ComponentType` base.

* Similarly, the `Module::getInterface` logic was changed to fall back to `Super::getInterface` for all the cases other than `IID_IModule`, so that it would be guaranteed to return the `IComponentType` at the same address as `ComponentType` in response to a `queryInterface`.

* Fixes for memory retain cycles

As part of the earlier change, I made the `Linkage` type hold a `RefPtr` to the `Session`.
The motivation there is it lets a user hang onto just a `slang::ISession` without having to also retain the `slang::IGlobalSession` for no immediately apparent reason.

There are two problems that this surfaced, one pre-existing and one new.

The new problem was that `Session` already held a `RefPtr<Linkage> m_builtinLinkage` for the linkage that holds the stdlib code.
I solved that problem by splitting the parent pointer in `Linkage` into two pointers: a raw pointer that is used to actually locate the parent session, and a ref-counted one that can be used to *optionally* retain the parent session.
The builtin linkage is then set up to explicitly not retain its parent, thus breaking the cycle.

The second problem was a pre-existing one, where every `ComponentType` was holding a retained pointer to its parent `Linkage`, but in turn the `Linkage` was holding retained pointers to many `ComponentTypes` (and subclasses thereof).
For this case I used the more expedient fix of making the parent pointer into a raw pointer, and figuring that it is a reasonable rule to expect user to retain the `Linkage` (aka `slang::ISession`) that owns a component type if they want to be able to use the component type.

I might need/want to investigate a better fix for the latter issue, but for now this seems to clean up the issues I was seeing in the tests. Fingers crossed.

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