DebugInfo: Handle emitting constants of C++ unicode character type.

Patch by Stephan Tolksdorf! (with some test case stuff by me)

Differential Revision: http://reviews.llvm.org/D3810

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209037 91177308-0d34-0410-b5e6-96231b3b80d8