[HLSL] `half` overloads and typedefs are supported when 16-bit types are not enabled

[llvm-beanz]

In HLSL when 16-bit types are not enabled, half overloads are all supported. DXC currently treats half as a distinct type from float in all contexts even if they are both 32-bit float types. This behavior can be preserved in Clang.

Acceptance Criteria

Clean up the existing test cases to allow half overloads and typedefs to be resolved without any additional flags required.