Improve the performance of the libc++ std::map formatter. This is not the full solution to the slowness of this formatter, but it's a 5% improvement in our testcase performance, which I am not going to complain too hard about.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@224373 91177308-0d34-0410-b5e6-96231b3b80d8