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

[via cfe-commits]

darkbuck wrote:

One motivation for this patch is to be able to mark individual inline asms with 'convergent.' So far, CUDA/HIP assumes all calls and inline-asms are convergent and marks all of them with 'convergent.' This guarantees correctness but loses possible optimizations for inline-asms with ALU instructions only.
After allowing 'convergent' to be a statement attribute, we could use 'convergent' mark only relevant inline-asms as convergent.

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