Recent builds of libcxx actually wrap an std::map's children values in a union containing either a member named __cc, or either of __cc and __nc (const vs. non-const). This level of wrapping is quite useless for LLDB to show to people, so try to detect it, and filter it out

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@217651 91177308-0d34-0410-b5e6-96231b3b80d8