C++: Use Token::utf16chars{Begin,End} where appropriate

...especially in CppTools/CppEditor where the offsets are used with a
QString/QTextDocument.

Change-Id: Ic6d18fbc01fb9cc899a9bd2d7424cd2edae487f1
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/libs/cplusplus/FindUsages.cpp

@@ -178,7 +178,7 @@ void FindUsages::reportResult(unsigned tokenIndex)
     if (col)
         --col;  // adjust the column position.
 
-    const int len = tk.bytes();
+    const int len = tk.utf16chars();
 
     const Usage u(_doc->fileName(), lineText, line, col, len);
     _usages.append(u);