Add slangc flag to `-zero-initialize` all variables (#3987)

* Default (zero'd) values with `-zero-initialize` flag

Adds `-zero-initialize` flag to set values to a __default() expression if they are missing a initExpr.

* address review and ensure __default calls ctor + zero's fields.

1. We must keep zero-initialize in SemanticsDeclHeaderVisitor. This is done because else a ctor will be initialized before we can set struct fields to `__default`.
2. IRDefaultCtorDecoration was added to track default ctor's with parent struct.
3. ParentAggTypeModifier was added to track ChildOfStruct->IRType for sharing data such as with functions. This is required to ensure we associate a lowered function with a lowered struct type

* Removed decoration to track defaultCtor in favor of field.

This was done since decorations are checked for IR objects, storing auxillary info does not work here as a result if usable object.

* address some review comments

Since `IDefaultInitializable` is taking a considerabley larger amount of time than anticipated I am pushing some of the other fixes requested. I did not remove the "IRStruct storing a default Ctor" hack yet.

mostly renamed/adjusted tests to work as intended

added test to ensure we don't synthisize a junk `= 0` when not in `zero initialize` mode

removed member in favor of sharedContext+dictionary.

* a working but incorrect impl

* default init without any IR hacks (fully working aside from generic/containored-types)

* Finish zero init code

1. IDefaultInitializer interface was added. If conforming, your type may be zero-initialized. To Conform a `__init()` is required
2. `[OnlyAutoInitIfForced]` was added. This attribute states that a default initializer should only be implicitly called if forced by the compiler (`zero-initialize` for example). This allows types which implicitly/explicitly conform to IDefaultInitialize to have optional auto-init behavior (which is Slang's default for user structs) to be disabled.

* note about `[OnlyAutoInitIfForced]`. This is required for std-lib to not automatically resolve init-expressions for std-lib, but it has the added benifit of allowing user made structs/classes to control the default behavior of initializing

* fix ErrType assumption

* testing why dx12 fails local but passes CI

* push vector changes to generic test

* push syntax adjustment, still figuring out what is wrong with cuda.

* remove debug changes & adjust style

* fix field-init expressions with structs initializers

don't init a static in a ctor. This would be illegal code and wrong code (init list in lower-to-ir)

* minor adjustments temporarily while the rest of the issue is discussed

* fix

* implement IDefaultInitializable

* remove a unneeded whitespace change

* fix type checking error

should be checking if a valid type is `Type`, not `BasicExpressionType`

* needs to be DeclRefType, not Type

* fix langguage server error

* change findinheritance for correctness + cleanup

* remove return false

verified the issue was `findInheritance`

* push attempt at language server fix

* still trying to fix inheritance

* added extension support, remove redundant code

Did not address all review comments yet, want to see if CI also passes my changes

* undo a change which caused CI to fail

* change logic  + DefaultConstructExpr

setup code to use defaultConstructExpr when possible to construct a default without overhead of invoke/related

also changed code so parent's defaultInitializable propegates to derived member

* 1. fix error in `isSubtype` 2. add flag to isSubtype

`subtypeInheritanceIsNotFullyResolved` was added since we may not be done the lookup stage but still require `isSubtype` checking to verify usage of inheritance while working with inheritance. In This case we will just skip `ensureLookup` and "caching" (since we don't have a cache invalidation system, nor need)

* fix bug in logic + add test to better catch the bug

* address comment + isSubTypeOption + wrapper type test,

* fix wrong code adjustment

I checked on the CI and realized I caused a failure, mistake was made not negating some code

* syntax, class naming capital

* remove stdlib default initialize changes, replace with `__default()` for init

* remove redundant code + fix defaultConstruct emitting

previously defaultConstruct emitting was crashing due to having generics unresolved. By not resolving the default construct immediately, everything works.

* remove a coment

* add test to ensure static variables dont `init` inside a struct's `__init`

* fix Ptr members breaking struct use

* address review and add -zero-initialize test

`-zero-initialize` test was added to be sure debug pointers are not broken with default init values

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Co-authored-by: Yong He <yonghe@outlook.com>

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