[clang] [HLSL] Default and Relaxed Availability Diagnostics (PR #92704)

[Chris B via cfe-commits]

https://github.com/llvm-beanz commented:

Looks pretty good to me. A few nits that you can take or leave.

The one real concern I have is that I actually don't like `getPrettyEnviromentName` being a string->string mapping. Is it possible to instead use the Triple environment enum?

https://github.com/llvm/llvm-project/pull/92704