[clang] [llvm] [clang] Add __builtin_elementwise_saturating_cast (PR #227603)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

For `__builtin_elementwise_add_sat` we had multiple targets that had exactly equivalent intrinsics that could be directly replaced with the generic - saturating cast is messier as theres so many different saturations that targets use (s->s, s->u, u->u just for x86).

https://github.com/llvm/llvm-project/pull/227603