[clang] [clang] Allow 'convergent' to be a statement attribute (PR #100637)

[Richard Smith via cfe-commits]

https://github.com/zygoloid commented:

Seems logical and useful to me.

This PR will need more tests: specifically, please make sure you have test coverage for the diagnostic, and for LLVM IR generation for both convergent calls and convergent inline asm.

Please also update the documentation for the attribute and the release notes.

https://github.com/llvm/llvm-project/pull/100637