[clang] [clang][bytecode] Implement the constexpr built-in abs function. (PR #112459)

[via cfe-commits]

c8ef wrote:

This issue arose when I was trying to address https://github.com/llvm/llvm-project/issues/51787, which concerns vector operations built-ins. I wanted to see how the single versions were implemented, only to discover that `__builtin_abs` cannot be evaluated in a constexpr context. So, I tried to fix it.

https://github.com/llvm/llvm-project/pull/112459