[clang] [clang][AMDGPU] Clean-up handling of named barrier type (PR #207687)

[Yaxun Liu via cfe-commits]

yxsamliu wrote:

@Pierre-vh Re: AMD SPIR-V named barrier

I checked the current ROCm SPIRV-LLVM-Translator. It only handles LLVM target extension types with `spirv.*` names. `target("amdgcn.named.barrier", ...)` would fall through to an unsupported path.

So I think it is better to diagnose this as unsupported in Clang for AMD SPIR-V for now, unless we also add explicit translator support.

https://github.com/llvm/llvm-project/pull/207687