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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Even with this fix, the behavior with mixed types still seems really confusing, especially if you mix signed/unsigned inputs.  Can we address that somehow?

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