Cleanup substitution of DeclaredSubtypeWitness.
Cleanup substitution of DeclaredSubtypeWitness.

Now using DeclaredSubtypeWitness::declRef to determine the proper argument index in a GenericSubstitution.

shader-slang/slang,shader-slang/slang,csyonghe/slang,shader-slang/slang,csyonghe/slang,csyonghe/slang