Move createCXString() functions out of CIndexer and into the clang::cxstring namespace.

We can much more succinctly refer to these functions this way.

Also change the default behavior of createCXString(StringRef&) to duplicate the
string.  This is almost always what we want.  The other case is where we pass
a constant c-string, which uses the other version of createCXString().

llvm-svn: 96423