Previously .fail.cpp tests for nodiscard were run with -Wunused-result
being a warning, not an error, when the compiler didn't support -verify.
When -verify isn't enabled this change judiciously adds -Werror=unused-result
when to only the failure tests containing the // expected-error string for nodiscard.
As a drive-by change, this patch also adds a missing // UNSUPPORTED: c++2a to
a test which was only supposed to run in C++ <= 11.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@322776 91177308-0d34-0410-b5e6-96231b3b80d8
