[cfi] Disable thinlto tests on Darwin.

These were added in r301016, but they're failing, because
-fsanitize=cfi seemingly causes -flto=thin to emit raw bitcode objects,
rather than the mach-o-wrapped bitcode we emit with -flto=thin alone.

That causes all tests to fail with ld64 errors.

Filed PR32741.

llvm-svn: 301065