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>

src/plugins/cpptools/projectpart.h

@@ -39,6 +39,8 @@
 
 #include <cplusplus/Token.h>
 
+#include <utils/cpplanguage_details.h>
+
 #include <QString>
 #include <QSharedPointer>
 
@@ -91,6 +93,7 @@ public:
     QString callGroupId;
 
     // Versions, features and extensions
+    ::Utils::Language language = Utils::Language::Cxx;
     ::Utils::LanguageVersion languageVersion = ::Utils::LanguageVersion::LatestCxx;
     ::Utils::LanguageExtensions languageExtensions = ::Utils::LanguageExtension::None;
     CPlusPlus::LanguageFeatures languageFeatures;