ClangTools: Fix state of the "Apply Fixits" button

* Disable the button when (re-)running an analysis.
* Ensure the button stays disabled when a file is saved.

Change-Id: I1140dd3938de554ea83d33c5bb490be3468d7db0
Reviewed-by: Cristian Adam <cristian.adam@qt.io>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/clangtools/clangtool.cpp

@@ -495,12 +495,13 @@ void ClangTool::startTool(ClangTool::FileSelection fileSelection,
     });
 
     m_perspective.select();
-
     m_diagnosticModel->clear();
 
-    setToolBusy(true);
     m_diagnosticFilterModel->setProject(project);
+    m_applyFixitsButton->setEnabled(false);
     m_running = true;
+
+    setToolBusy(true);
     handleStateUpdate();
     updateRunActions();