[libc++] Get rid of `-Xclang -verify-ignore-unexpected=error` in the test suite

[ldionne]

We use -Xclang -verify-ignore-unexpected=error to ignore unrelated failures in some of our Clang-verify tests, however the result is that those verify tests will pass if the code fails to compile for any reason at all. That's really not what we intend. Originally found by @dalg24 .