Fix PR 19663. Some calls to find(vector<bool>) were returning iterators that were subtly invalid (didn't compare equal). Thanks to Erik Verbruggen for the report (and diagnosis)

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@208096 91177308-0d34-0410-b5e6-96231b3b80d8