[HLSL] Disallow 16-bit integer types if native 16-bit types are disabled

[llvm-beanz]

DXC disallows declarations of 16-bit types if the native 16-bit types are disabled. In Clang, we #ifdef out the 16-bit intrinsics to prevent generating 16-bit types.

Acceptance Criteria

Test cases that produce diagnostics when users use 16-bit types without enabling native 16-bit types.

[spall]

https://godbolt.org/z/cjndaT4qM
https://godbolt.org/z/46csPzsM4