[libc++] Fix handling of negated character classes in regex

Summary:
This commit fixes a regression introduced in r316095, where we don't match
inverted character classes when there's no negated characrers in the []'s.

rdar://problem/43060054

Reviewers: mclow.lists, timshen, EricWF

Subscribers: christof, dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D50534

llvm-svn: 340609