[clang] [HLSL] standardize builtin unit tests (PR #83340)

[Greg Roth via cfe-commits]

pow2clk wrote:

Do we have the suggestions that this is responding to written down somewhere? I think it would be useful to have those guidelines for anyone who might want to contribute HLSL tests. At any rate, I'd like to know the ones that this is in response to. 

https://github.com/llvm/llvm-project/pull/83340