Clang: Centralize job related switch code

...and where applicable, ensure that a warning is printed once a
jobrequest type is not handled.

Change-Id: Ida0b5a175947ebf75c1c4e5116a77f0270825336
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/tools/clangbackend/ipcsource/clangjobrequest.h

@@ -39,7 +39,9 @@
 
 namespace ClangBackEnd {
 
+class ClangCodeModelClientInterface;
 class Document;
+class IAsyncJob;
 
 class JobRequest
 {
@@ -91,6 +93,9 @@ public:
 
     JobRequest();
 
+    IAsyncJob *createJob() const;
+    void cancelJob(ClangCodeModelClientInterface &client) const;
+
     bool operator==(const JobRequest &other) const;
 
 public: