Clang: Clean up diagnostic widget

* Use a single QLabel - No need for all the QLabels we used. Also, a
  single QLabel enables selecting and copying all the diagnostics, which
  is handy when displayed in the info bar.
* Avoid call to Utils::ToolTip::hideImmediately() if the location is
  clicked from the info bar.
* Simplify code and API

Change-Id: Ib991364e4d6f40ef02dada8ebbb90fe6ff8ae1a1
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/libs/clangbackendipc/diagnosticcontainer.h

@@ -144,6 +144,11 @@ public:
             && first.location_ == second.location_;
     }
 
+    friend bool operator!=(const DiagnosticContainer &first, const DiagnosticContainer &second)
+    {
+        return !(first == second);
+    }
+
 private:
     SourceLocationContainer location_;
     QVector<SourceRangeContainer> ranges_;