Cleanup quick_exit tests and get them passing in C++03.

Wrap the original test in _LIBCPP_HAS_QUICK_EXIT so it only runs when we have
quick_exit and add two new tests that check that when _LIBCPP_HAS_QUICK_EXIT
is not defined then no definition of std::at_quick_exit or std::quick_exit are
available.

llvm-svn: 222298