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

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

I expect these kind of builtins to map directly to an IR construct, but this appears to be directly emitting the logic in clang 

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