[libc++] Add yet another test for inverted character classes

This was reported as part of a bug report that ended up being a
duplicate for r340609, but I'm adding the test case since it's
ever so slightly different from what we had before.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@370109 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed