[clang] Fix typos in the default logic for CLANG_DEFAULT_UNWINDLIB

CLANG_DEFAULT_RTLIB had a typo, and libunwind isn't a valid
option for it.

This keeps the actual behaviour from before, defaulting to none if
using compiler-rt as rtlib.

Differential Revision: https://reviews.llvm.org/D98022

GitOrigin-RevId: 002dd47bdd674fad8186650f07458b1e062545df
1 file changed