[clang] [llvm] [IR] Mark convergence intrins as has-side-effect (PR #134844)

[Sameer Sahasrabuddhe via cfe-commits]

ssahasra wrote:

>From the spec for convergence control tokens:
https://llvm.org/docs/ConvergentOperations.html#inferring-non-convergence

> An optimizer may remove the convergent attribute on a function if it can prove that the function does not contain a call to `llvm.experimental.convergence.entry`, or any uncontrolled convergent operations.

https://github.com/llvm/llvm-project/pull/134844