[libclc] [libclc] Use native spirv*-mesa-mesa3d clang triple for mesa (PR #199618)

[Konrad Kleine via cfe-commits]

https://github.com/kwk approved this pull request.

I've read all comments and I can see why `mesa` doesn't have to be explicitly called out in the triple. But implementation wise I think it makes sense to have it for clang to be able to differentiate SPIRV triples for different vendors/OSes. Therefore I approve this.

https://github.com/llvm/llvm-project/pull/199618