<rdar://problem/12239827>

Providing a data formatter for libc++ std::wstring
In the process, refactoring the std::string data formatter to be written in C++ so that commonalities between the two can be exploited
Also, providing a new API on the ValueObject to navigate a hierarchy by index-path
Lastly, an appropriate test case is included



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@172282 91177308-0d34-0410-b5e6-96231b3b80d8