Fix bug 20740 - std::set/std::map don't support heterogeneous lookup for count(). Thanks to Jim Porter for the bug report

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@216353 91177308-0d34-0410-b5e6-96231b3b80d8
Merge branch 'master' of http://llvm.org/git/libcxx