According to UTF-8 encoding rules, there are no valid one byte long codepoints which start with a bigh bit set (i.e. 0x80 or higher value). Hence such XML output needs to be escaped, same as a control character.
This allows reducing the amount of friends needed for its interface
and some extra tricks later.
The bad part is that the pointer can become invalidated via
calls to other StringRef's public methods, but c'est la vie.
