[clang] [llvm] [clang] Add spir_kernel attribute (PR #137882)

[Nick Sarnie via llvm-commits]

sarnex wrote:

@AaronBallman I just pushed a first attempt to combine the attrs, however I'm not sure if it's much cleaner given the attrs have different subjects/allowed cases/expected warnings/etc. Do you mind taking a first look and seeing if you think the general direction is a good idea? Thanks

https://github.com/llvm/llvm-project/pull/137882