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

[Timm Baeder via cfe-commits]

https://github.com/tbaederr requested changes to this pull request.

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

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