C++: Fix completion for doxygen tags I

There are three cases that must be handled:

 1. Completion in C++ style comment
 2. Completion in first line of a C style comment
 3. Completion in non-first line of a C style comment

This change fixes case 1 + 2. Case 3 will be addressed in a follow-up
change, same goes for the duplication.

Task-number: QTCREATORBUG-15143
Change-Id: I449711f965ddcbbe6158870a8a5ae33218e0d238
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>

src/plugins/cpptools/cppcompletionassistprocessor.cpp

@@ -70,4 +70,10 @@ void CppCompletionAssistProcessor::addSnippets()
     m_completions.append(m_snippetCollector.collect());
 }
 
+bool CppCompletionAssistProcessor::isDoxygenTagCompletionCharacter(const QChar &character)
+{
+    return character == QLatin1Char('\\')
+        || character == QLatin1Char('@') ;
+}
+
 } // namespace CppTools