PchManager: Split pch tasks in project and system pch tasks

Like you can see in the task numbers this patch is touching many different
areas. So I will only touch the main parts. It is using a clang action
instead of an extra process which will be enabling the handling of
generated files in PCHs. The flags from the project part are now not
anymore transformed in a command line but they are saved in the container
semantically aware so that they can later be merged. Most of this patch is
simply polishing of other patches.

Task-number: QTCREATORBUG-21346
Task-number: QTCREATORBUG-21380
Task-number: QTCREATORBUG-21382
Task-number: QTCREATORBUG-21383
Task-number: QTCREATORBUG-21693
Task-number: QTCREATORBUG-21778
Change-Id: I9b0c02d8149b554254e819448fbc61eeaa5b7494
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

tests/unit/unittest/refactoringprojectupdater-test.cpp

@@ -52,7 +52,7 @@ MATCHER_P(IsProjectPartContainer, projectPartId,
           std::string(negation ? "hasn't" : "has")
           + " name " + std::string(projectPartId))
 {
-    const ClangBackEnd::V2::ProjectPartContainer &container = arg;
+    const ClangBackEnd::ProjectPartContainer &container = arg;
 
     return  container.projectPartId == projectPartId;
 }