ClangPchManager: Introduce PchTaskQueue

With the PchTaskQueue the pipeline is almost complete.

Task-number: QTCREATORBUG-21346
Change-Id: I5f05d525db1679eb37dd1d462076c1ed42958099
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/tools/clangrefactoringbackend/source/symbolindexing.h

@@ -105,7 +105,7 @@ public:
         m_indexerScheduler.disable();
         while (!m_indexerScheduler.futures().empty()) {
             m_indexerScheduler.syncTasks();
-            m_indexerScheduler.freeSlots();
+            m_indexerScheduler.slotUsage();
         }
     }