[libc++] Fix XFAILs on macOS when exceptions are disabled

Some tests are marked as failing on platforms where the dylib does not
provide the required exception classes. However, when testing with
exceptions disabled, those tests shouldn't be marked as failing.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@353210 91177308-0d34-0410-b5e6-96231b3b80d8