Display 'static_assert failed: message' instead of
'static_assert failed "message"' to be consistent
with other implementations and be slightly more
readable.
Reviewed By: #libc, aaron.ballman, philnik, Mordante
Differential Revision: https://reviews.llvm.org/D128844
Instead of using `.fail.cpp` tests, use `.verify.cpp` to check for the
exact reason of the failure. In the case of deduction guides, use SFINAE
based tests instead since that is our preferred way of testing those.
Finally, ensure that we actually run the test in `iter_alloc_deduction.pass.cpp`,
since we were not running anything before.
Differential Revision: https://reviews.llvm.org/D123055