<rdar://problem/12725746>

Providing data formatters for char16_t* and char32_t* C++11-style Unicode strings
Using this chance to refactor the UTF data reader used for data formatters for added generality
Added a relevant test case



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@172119 91177308-0d34-0410-b5e6-96231b3b80d8