ClangStaticAnalyzer: Really cancel on user request

Currently when clicking the X icon next to Clang Static Analyzer
progress bar, clang is no longer executed, but the mouse cursor remains
busy, and trying to quit Qt Creator warns about running analyzer.

Change-Id: I1dc8b92fdaf03cd25a2ad40d24d0f76907dd791c
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>