[clang] [clang] Fix sub-integer __builtin_elementwise_(add|sub)_sat (PR #119423)

[Fraser Cormack via cfe-commits]

frasercrmck wrote:

I just realised this is also a problem with the `bitreverse` and `popcount` builtins on signed types for the same reason.

https://github.com/llvm/llvm-project/pull/119423