[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.

llvm-svn: 353215
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: a53eb79be683120402e7e57aebb47b36fc259066
3 files changed