Cpp{Tools,Editor}: Respect multi-QChar code points when handling identifiers

* Consolidate code dealing with C++ identifiers into cpptoolsreuse.h
* Handle code points that are represented with two QChars

Change-Id: I4fb4435aa539f65d88598cac0b50629f33f32440
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/cppeditor/cpphighlighter.cpp

@@ -412,7 +412,7 @@ void CppHighlighter::highlightDoxygenComment(const QString &text, int position,
             ++it;
 
             const QChar *start = it;
-            while (it->isLetterOrNumber() || it->unicode() == '_')
+            while (CppTools::isValidAsciiIdentifierChar(*it))
                 ++it;
 
             int k = CppTools::classifyDoxygenTag(start, it - start);