Clang: Add semantic C++ operator-token styling

We used to style overloaded operators in the same way
as C++'s built-in operators. There was no way to tell
if a + token would call a operator+() function or not.

Now, if an operator is overloaded (redefined),
we give it the "Overloaded Operator"-mixin so users
can style it differently.

Note: Calls to overloaded 'new' and 'delete' are not
highlighted by "Overloaded Operator". This is because
clang today always maps these to CXCursor_CXXNewExpr
and CXCursor_CXXDeleteExpr with cursor.spelling == ""
(empty string). So there is no (?) quick way for us
to tell if a new/delete-token was overloaded or not.

After follow-ups, follow symbol will work for operator
overload usages in current translation unit.
Commit is appended by Ivan Donchevskii.

Task-number: QTCREATORBUG-19659
Change-Id: I157855d482a61ad2059642a1ee982089fcb7d312
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/texteditor/texteditorconstants.cpp

@@ -63,6 +63,7 @@ const char *nameForStyle(TextStyle style)
     case C_KEYWORD:             return "Keyword";
     case C_PRIMITIVE_TYPE:      return "PrimitiveType";
     case C_OPERATOR:            return "Operator";
+    case C_OVERLOADED_OPERATOR: return "Overloaded Operator";
     case C_PREPROCESSOR:        return "Preprocessor";
     case C_LABEL:               return "Label";
     case C_COMMENT:             return "Comment";