Clang: Avoid storing settings more than needed

There is no need to store on every key stroken when changing the
diagnostic config in the projects mode.

Change-Id: I627c11621b999527c110862365ba5c2a0b1e878b
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangprojectsettingswidget.h

@@ -51,6 +51,7 @@ private:
     void onCustomWarningConfigsChanged(const CppTools::ClangDiagnosticConfigs &customConfigs);
     void onDelayedTemplateParseClicked(bool);
     void onClangSettingsChanged(int index);
+    void onAboutToSaveProjectSettings();
     void refreshDiagnosticConfigsWidgetFromSettings();
     void connectToCppCodeModelSettingsChanged();
     void disconnectFromCppCodeModelSettingsChanged();