Initial implementation for decl-tree reflection API (#4666)

* Initial implementation for decl-tree reflection API

This patch adds Slang API methods for walking all the declarations in the AST.

We expose this functionality through an abstract `DeclReflection` class that can be a type, function or a variable declaration.
We also provide ways to cast the decl to a `FunctionReflection`, `TypeReflection` or `VariableReflection` and traverse through the child nodes (for instance, a struct type will have component variable declarations)

This patch also adds `ISlangInternal` as an internal COM interface to allow us to cast IGlobalSession to the internal Session pointer while bypassing any wrappers (such as the capture interface)

* Update slang.h

* Remove `ISlangInternal` (its causing a diamond pattern w.r.t `ISlangUnknown`) and use `ComPtr` for proper ref management.

* Update unit-test-decl-tree-reflection.cpp

* Change `FunctionDeclBase` to use `DeclRef` instead of directly using the decl.

* Update slang-reflection-api.cpp

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