TaskTree: Get rid of Storage subclass of GroupItem

Make the c'tor of the GroupItem taking the TreeStorageBase
public instead.

This addresses the 20th point in the master task below.

Task-number: QTCREATORBUG-28741
Change-Id: I78320ec24b4e4b915ab549135fb93b45c658b9f2
Reviewed-by: hjk <hjk@qt.io>

src/plugins/clangcodemodel/clangdlocatorfilters.cpp

@@ -192,7 +192,7 @@ static LocatorMatcherTask currentDocumentMatcher()
     };
 
     const Group root {
-        Tasking::Storage(resultStorage),
+        resultStorage,
         CurrentDocumentSymbolsRequestTask(onQuerySetup, onQueryDone, CallDoneIf::Success),
         AsyncTask<void>(onFilterSetup)
     };