[clang] [clang][SPIR-V] Use the C calling convention as the target default (PR #210882)

[Tom Honermann via cfe-commits]

tahonermann wrote:

One more minor request after all if you don't mind. In `clang/include/clang/Basic/Specifiers.h`, `CC_SpirFunction` is declared with a comment that will be incorrect following this change.
https://github.com/llvm/llvm-project/blob/c863ebf68ce9fdcb33958778fb53f444c33bb9e0/clang/include/clang/Basic/Specifiers.h#L292
Can you please remove or update that comment? It looks to me like a follow on PR could remove `CC_SpirFunction` completely, assuming there is no motivation to add an attribute to enable explicitly declaring functions with that calling convention.

https://github.com/llvm/llvm-project/pull/210882