Fix nodiscard failure tests on compilers w/o -verify.

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.

llvm-svn: 322776