NSSet formatter is now C++ code
Split some NS* formatters in their own source files
Refactored a utility function for the C++ formatters to use
Fixed the skip-summary test case to be explicit about requiring libstdc++ for operation



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@175323 91177308-0d34-0410-b5e6-96231b3b80d8