Clang: Do not show global completions after comma

Handle the case when we try to show the function hint but
do not find any completion. Do not fall back to the normal
code completion in such case.

Fixes: QTCREATORBUG-21624
Change-Id: I147d71b8970c18d49947f68786347a9db97736bb
Reviewed-by: Marco Bubke <marco.bubke@qt.io>

src/plugins/clangcodemodel/clangcompletionassistprocessor.h

@@ -97,6 +97,7 @@ private:
     CompletionRequestType m_sentRequestType = NormalCompletion;
     bool m_requestSent = false;
     bool m_addSnippets = false; // For type == Type::NormalCompletion
+    bool m_fallbackToNormalCompletion = true;
 };
 
 } // namespace Internal