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

[Nick Sarnie via cfe-commits]

sarnex wrote:

> Do we need yet another calling convention or is there a way we can start to combine some of these for all the offloading languages? It seems we keep re-implementing the same concepts multiple times for each language and it would be nice to share as much of the frontend logic as possible rather than keep doing this dance of adding another case to every switch in the compiler. :-D

Do you have an idea? Maybe we could have a `calling_convention` attribute that takes in a string that specifies the calling convention?

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