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/texteditorsettings.cpp

@@ -226,7 +226,8 @@ TextEditorSettings::TextEditorSettings()
                              tr("Reserved keywords of the programming language except "
                                 "keywords denoting primitive types."), Qt::darkYellow);
     formatDescr.emplace_back(C_OPERATOR, tr("Operator"),
-                             tr("Operators (for example operator++ or operator-=)."));
+                             tr("Non user-defined language operators.\n"
+                                "To style user-defined operators, use Overloaded Operator."));
     formatDescr.emplace_back(C_PREPROCESSOR, tr("Preprocessor"),
                              tr("Preprocessor directives."), Qt::darkBlue);
     formatDescr.emplace_back(C_LABEL, tr("Label"), tr("Labels for goto statements."),
@@ -313,6 +314,10 @@ TextEditorSettings::TextEditorSettings()
                              QColor(255, 190, 0),
                              QTextCharFormat::DotLine,
                              FormatDescription::ShowUnderlineControl);
+    formatDescr.emplace_back(C_OVERLOADED_OPERATOR,
+                             tr("Overloaded Operators"),
+                             tr("Calls and declarations of overloaded (user-defined) operators."),
+                             Format::createMixinFormat());
     Format declarationFormat = Format::createMixinFormat();
     declarationFormat.setBold(true);
     formatDescr.emplace_back(C_DECLARATION,