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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

If people aren't using the edge cases, they won't notice if we start error'ing out.  If they are, it's pretty easy to adapt the code to build with both old and new compilers.  A quick GitHub search shows exactly one user of the builtins outside of clang itself, and that code uses a vector input.

Maybe makes sense to RFC it for wider exposure, but I doubt anyone will object.

We should probably write a release note for this in any case.

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