[Clang] Add elementwise saturated add/sub builtins

This patch implements `__builtin_elementwise_add_sat` and `__builtin_elementwise_sub_sat` builtins.

These map to the add/sub saturated math intrinsics described here:
https://llvm.org/docs/LangRef.html#saturation-arithmetic-intrinsics

With this in place we should then be able to replace the x86 SSE adds/subs intrinsics with these generic variants - it looks like other targets should be able to use these as well (arm/aarch64/webassembly all have similar examples in cgbuiltin).

Differential Revision: https://reviews.llvm.org/D117898