test: Allow using clang -verify for failures rather than exit 1

Currently, failure tests work by checking that compilation exits 1.
This can lead to tests that fail for the wrong reason, so it'd be
preferable to convert them to check for specific errors.

This adds use_clang_verify parameter that runs failure tests using
clang's -verify flag. I'll convert some tests in subsequent commits,
and once all of the tests are converted we should key this on whether
cxx_under_test is clang.

I've also converted one of the unique.ptr tests, since it's the one
that motivated the idea of using clang -verify when possible in the
review of r216317.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@217009 91177308-0d34-0410-b5e6-96231b3b80d8