[clang] [llvm] [mlir] [NVPTX] Switch front-ends and tests to ptx_kernel cc (PR #120806)

[Artem Belevich via llvm-commits]

Artem-B wrote:

> A 2% improvement is an excellent result!

LLVM caches these attributes these days. Before we've implemented that, nvvm metadata lookups did take a lot of time.

That said, CC and attributes are a better way to represent the info, and getting rid of the metadata is worthwhile even if does not speed up the compilation.

https://github.com/llvm/llvm-project/pull/120806