CppTools: Cancel parsing if editor is closed

The m_parserFuture.cancel() in ~BuiltinEditorDocumentProcessor() did not
cancel anything. Thus, closing a document while the parser was running
led to a blocking UI thread.

Now it cancels at the next include directive it encounters.

Change-Id: I092fddbbd747e0bc95265b6e9b4fcc26b3f76cb3
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/clangcodemodel/clangeditordocumentparser.cpp

@@ -35,7 +35,8 @@ ClangEditorDocumentParser::ClangEditorDocumentParser(const QString &filePath)
     setConfiguration(config);
 }
 
-void ClangEditorDocumentParser::updateHelper(const CppTools::WorkingCopy &)
+void ClangEditorDocumentParser::updateHelper(const QFutureInterface<void> &,
+                                             const CppTools::WorkingCopy &)
 {
     State state_ = state();
     state_.projectPart = determineProjectPart(filePath(), configuration(), state_);