TextEditor: Introduce shortcut for forcing a function hint proposal

... and support it in the ClangCodeModel.
This allows users to get function signature(s) displayed regardless of
where exactly the cursor is on the function call.

Fixes: QTCREATORBUG-19394
Change-Id: I033e8774db93680bfc3ee52610b817e0ef8ccc76
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangcompletioncontextanalyzer.h

@@ -51,7 +51,8 @@ public:
         CompleteIncludePath,
         CompletePreprocessorDirective,
         CompleteSignal,
-        CompleteSlot
+        CompleteSlot,
+        CompleteNone
     };
     CompletionAction completionAction() const { return m_completionAction; }
     unsigned completionOperator() const { return m_completionOperator; }