Clang: Fix canceling of clang query

Every AST unit is created and queried asynchronously, like before, but
we don't wait anymore but use a timer to process new sources. So the server
will not be blocked and can process other messages like cancel.

Change-Id: If0e69466c78f628190f59fd32a03cab1c3a4d0a3
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/clangrefactoring/qtcreatorsearchhandle.cpp

@@ -59,6 +59,7 @@ void QtCreatorSearchHandle::cancel()
 {
     SearchHandle::cancel();
     promise.reportCanceled();
+    promise.reportFinished();
 }
 
 void QtCreatorSearchHandle::finishSearch()