Add a UTF8 to UTF16 conversion wrapper for use in the pdb dumper

This can also be used instead of the WindowsSupport.h ConvertUTF8ToUTF16
helpers, but that will require massaging some character types. The
Windows support routines want wchar_t output, but wchar_t is often 32
bits on non-Windows OSs.

llvm-svn: 227122