[clang] [HIP] diagnose -mwavefrontsize64 for gfx10+ (PR #140185)

[Yaxun Liu via cfe-commits]

yxsamliu wrote:

> I don't think we should be introducing an additional option that behaves identically to another option just differing by a warning. This also isn't really a problem with the language, but the library support. The actual codegen should work just fine. Is there some other existing mechanism to just silence the warning?

We could emit a warning for -mwavefrontsize64 for HIP on gfx10+, which can be disabled by -Wno-unsupported-wave64. Then we do not need introduce -mforce-unsafe-wavefrontsize64.

@b-sumner are you OK with this?

https://github.com/llvm/llvm-project/pull/140185