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

[Nick Sarnie via cfe-commits]

sarnex wrote:

> I was thinking that orthogonal calling conventions could be combined so that we need fewer of them. e.g., if spir_kernel cannot be mixed with nvptx_kernel in the same TU, then there's no need for two distinct internal representations for the attributes, just two distinct spellings that are based on the target.

Ah, I see. 

Okay, I'll try, but it might take me a while. Thanks for the feedback. Marking this as a draft for now.

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