[clang] [clang] Constant evaluation support for `__builtin_copysign{,f,l}` (PR #225167)

[Lucas Mellone via cfe-commits]

lknknm wrote:

> `AST/ByteCode/InterpBuiltin.cpp` needs the same change.

Done, please check if it's enough. Mind you I tagged other people for review  because I didn't know who specifically was responsible for it. Up to the maintainers to choose to merge it right away or not. Thanks!

https://github.com/llvm/llvm-project/pull/225167