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/symbolstorage.h

@@ -332,7 +332,7 @@ public:
         m_database
     };
     mutable ReadStatement m_getPrecompiledHeader{
-        "SELECT pchPath, pchBuildTime FROM precompiledHeaders WHERE projectPartId = ?",
+        "SELECT projectPchPath, projectPchBuildTime FROM precompiledHeaders WHERE projectPartId = ?",
         m_database
     };
 };