Clang: Take over jobs if document's project changes

On registerProjectPartsForEditor() we recreated the affected
DocumentProcessors, but did not take care of the jobs that were in the
queue, which effectively could led to lost jobs. Catch up on this.

Task-number: QTCREATORBUG-18856
Change-Id: I4184e5dc6480667953f2d2081ccf39a28c092186
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/tools/clangbackend/source/clangjobrequest.h

@@ -96,6 +96,7 @@ public:
 
     IAsyncJob *createJob() const;
     void cancelJob(ClangCodeModelClientInterface &client) const;
+    bool isTakeOverable() const;
 
     bool operator==(const JobRequest &other) const;