Remove tests that va_copy is not defined when C++ < 11.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@222405 91177308-0d34-0410-b5e6-96231b3b80d8
Remove xfail tag for darwin from quick_exit test

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@222406 91177308-0d34-0410-b5e6-96231b3b80d8