[libc++] Const qualify __gnu_cxx::hash_map<>::const_iterator::pointer type.

Differential Revision: http://llvm-reviews.chandlerc.com/D2811

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@202749 91177308-0d34-0410-b5e6-96231b3b80d8