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

[via cfe-commits]

darkbuck wrote:

> We should be removing the source level convergent attribute, not adding more uses of it. It was a mistake to define it in the positive direction, and it is completely unusable for end users. Every piece of code that transitively calls the function in the entire program must also be marked convergent. The only way to practically achieve this is if the target/language has convergent operations, every operation must be marked convergent by default (i.e. the ConvergentFunctions language property should be set)
> 
> For optimization hints, we could have a source level noconvergent attribute to opt-out, which would be the inverse of this.
> 
> On the IR level we should pick https://reviews.llvm.org/D69498 back up.

I agree with and revise PR to replace it with `noconvergent`. For non-GPU languages, it's just a NOP. For CUDA/HIP, we will skip adding `convergent` attribute.

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