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

…on-to-zero-initialize-all-variables

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

…on-to-zero-initialize-all-variables

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

…hub.com/ArielG-NV/slang into option-to-zero-initialize-all-variables

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.

…on-to-zero-initialize-all-variables

…ontainored-types)

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

…on-to-zero-initialize-all-variables

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

…hub.com/ArielG-NV/slang into option-to-zero-initialize-all-variables

…hub.com/ArielG-NV/slang into option-to-zero-initialize-all-variables

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

verified the issue was `findInheritance`

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

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

`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)

…hub.com/ArielG-NV/slang into option-to-zero-initialize-all-variables

…hub.com/ArielG-NV/slang into option-to-zero-initialize-all-variables

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

…for init

…hub.com/ArielG-NV/slang into option-to-zero-initialize-all-variables

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

…init`

…hub.com/ArielG-NV/slang into option-to-zero-initialize-all-variables

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