[libc++] Require the use of clang-verify in .fail.cpp tests that don't fail without it

Some tests do not fail at all when -verify is not supported, unless some
arbitrary warning flag is added to make them fail. We currently used
-Werror=unused-result to make them fail, but doing so makes the test
suite a lot more inscrutable. It seems better to just disable those
tests when -verify is not supported.

Differential Revision: https://reviews.llvm.org/D76256