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/tools/clangrefactoringbackend/source/clangrefactoringbackend-source.pri

@@ -2,6 +2,7 @@ INCLUDEPATH += $$PWD
 
 HEADERS += \
     $$PWD/clangrefactoringbackend_global.h \
+    $$PWD/clangquerygatherer.h
 
 !isEmpty(LIBTOOLING_LIBS) {
 SOURCES += \
@@ -31,3 +32,6 @@ HEADERS += \
     $$PWD/sourcerangeextractor.h \
     $$PWD/locationsourcefilecallbacks.h
 }
+
+SOURCES += \
+    $$PWD/clangquerygatherer.cpp