[clang] [HLSL] Implement HLSL Flat casting (excluding splat cases) (PR #118842)

[Sarah Spall via cfe-commits]

spall wrote:

> One small request: can you add a test that verifies we produce an error if you try to cast to or from a structure containing a bitfield?

I think I already did this clang/test/SemaHLSL/Language/ElementwiseCast-errors.hlsl

https://github.com/llvm/llvm-project/pull/118842