[Feature Request] Add SubObject attribute

[bob80905]

Is your feature request related to a problem? Please describe.
Subobjects may exist in library shaders without any function declarations. If subobjects exist in such a library shader, they won't be semantically diagnosed in the translation unit semantic analysis. This means if subobjects use a value that has an availability attribute in its initializer, it won't be caught.

Describe the solution you'd like
We need a way to detect that a given decl is a subobject decl, to know whether or not to investigate further and check if its initializer contains any availability attributes. A proposed solution is to add a subobject hlsl attribute, that will be attached to all subobject decls.

This will help with the OMM implementation, since RayTracingPipelineConfig1 subobjects may be initialized with values that have availability attributes.