Clangd: Handle settings changes

Users no longer have to re-load a project for settings changes to take
effect.

Change-Id: I86dccccac14a30514c8dac292c7765ee4806f6ba
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangmodelmanagersupport.h

@@ -114,6 +114,7 @@ private:
 
     void onProjectPartsUpdated(ProjectExplorer::Project *project);
     void onProjectPartsRemoved(const QStringList &projectPartIds);
+    void onClangdSettingsChanged();
 
     void onDiagnosticConfigsInvalidated(const QVector<::Utils::Id> &configIds);