unwind: remove an instance of -Wexpansion-to-defined

This follows the pattern in the Apple clause duplicating a tuple of definitions.
However, it will define them to a value rather than a defined check to remove
the `-Wexpansion-to-defined` warning (which may be treated as an error).

This also opens the door to unifying the two code paths into one.

NFC.

llvm-svn: 266915