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/libs/cplusplus/pp-engine.h

@@ -56,6 +56,8 @@
 #include <QByteArray>
 #include <QPair>
 
+#include <functional>
+
 namespace CPlusPlus {
 
 class Environment;
@@ -81,6 +83,9 @@ public:
     QByteArray run(const QString &filename, const QByteArray &source,
                    bool noLines = false, bool markGeneratedTokens = true);
 
+    using CancelChecker = std::function<bool()>;
+    void setCancelChecker(const CancelChecker &cancelChecker);
+
     bool expandFunctionlikeMacros() const;
     void setExpandFunctionlikeMacros(bool expandFunctionlikeMacros);
 
@@ -253,6 +258,7 @@ private:
     Client *m_client;
     Environment *m_env;
     QByteArray m_scratchBuffer;
+    CancelChecker m_cancelChecker;
 
     bool m_expandFunctionlikeMacros;
     bool m_keepComments;