[libc++] Fix XFAILs when exceptions are disabled

It turns out that I un-XFAILed too many tests in r353210: some tests
actually fail whether exceptions are enabled or not because they use
types that are marked as unavailable even when exceptions are disabled.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@353215 91177308-0d34-0410-b5e6-96231b3b80d8