[libc++][numeric] Marked saturation artithmetic functions as `[[nodiscard]]`#166898

Zingam merged 2 commits intollvm:mainllvm/llvm-project:mainfrom

H-G-Hristov:hgh/libcxx/nodiscard-to-saturation-arithmeticH-G-Hristov/llvm-project:hgh/libcxx/nodiscard-to-saturation-arithmeticCopy head branch name to clipboard