Implement GetElementCount for TypeCastIntVal

[aidanfnv]

Fixes #8894

Adds handling of TypeCastIntVal in GetElementCount, recursively calling GetElementCount on its base IntVal, allowing a SpecializationConstant to be used as an array size.

Also adds a check for specialization constants in GetElementCount so that if a DeclRefIntVal has a VarDeclBase with the SpecializationConstantAttribute, GetElementCount will return LayoutSize::infinite() to ensure that the array gets a binding but does not get a size at compile-time. In this case the DeclRefIntVal is the base IntVal of the TypeCastIntVal that we recursively check.

[kaizhangNV]

LGTM

[aidanfnv]

The recursive call to GetElementCount returns 0 for SpecializationConstants, which means that SpecializationConstant-sized arrays do not get binding layouts, which leads to the reproducer shader (and the test based on it) to fail SPIR-V validation due to the missing binding.
Since the specialization constant's value can be changed at runtime (the value set in the shader, e.g. MAX_SAMPLES = 32, is just a default), we cannot ensure the array's size at compile time, so we must treat it as unbounded at that time.

I have added a check for specialization constants in GetElementCount so that if a DeclRefIntVal has a VarDeclBase with the SpecializationConstantAttribute, GetElementCount will return LayoutSize::infinite() to ensure that the array gets a binding but does not get a size at compile-time. In this case the DeclRefIntVal is the base IntVal of the TypeCastIntVal that we recursively check.

[kaizhangNV]

This is unnecessarily true, if downstream doesn't provide the value via pipeline, it will just use the default value. So I don't think this change is correct.

[kaizhangNV]

Why do we need to differentiate it from generic parameter?

Both of them cannot be determined at front-end, so I don't know why there is difference.