[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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@340609 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/re/re.alg/re.alg.search/invert_neg_word_search.pass.cpp

@@ -18,7 +18,7 @@
 
 #include <regex>
 #include <cassert>
-#include "test_macros.h"
+
 
 // PR34310
 int main()