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

[via cfe-commits]

c8ef wrote:

> > This patch fixes the issue.
> 
> What issue? Is there a bug open about this?
> 
> You didn't implement it for the bytecode interpreter at all, only for the current interpreter.

Yes, I realized that. Apologies for the oversight. I will add the implementation to `InterpBuiltin.cpp`. Also, I'm curious why this triggers UBSan. Does the patch itself have a bug?

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