Clang: Use CppHoverHandler for diagnostic tooltips

We used to call QTextCharFormat::setToolTip from the ExtraSelection to
install the diagnostic tooltip. Since this allows to set only text
tooltips and we would like to introduce a custom tooltip widget for
diagnostics, make use of CppHoverHandler, which is more flexible.

Change-Id: Ia1b2c3c50810596ce4a3a025002e6e4efd8789db
Reviewed-by: Alessandro Portale <alessandro.portale@theqtcompany.com>

src/plugins/clangcodemodel/clangeditordocumentprocessor.h

@@ -76,6 +76,12 @@ public:
     TextEditor::QuickFixOperations
     extraRefactoringOperations(const TextEditor::AssistInterface &assistInterface) override;
 
+    bool hasDiagnosticsAt(uint line, uint column) const override;
+    void showDiagnosticTooltip(const QPoint &point,
+                               QWidget *parent,
+                               uint line,
+                               uint column) const override;
+
     ClangBackEnd::FileContainer fileContainerWithArguments() const;
 
     void clearDiagnosticsWithFixIts();