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/cpptools/cppbuiltinmodelmanagersupport.h

@@ -41,6 +41,7 @@ public:
     ~BuiltinModelManagerSupport() override;
 
     CppCompletionAssistProvider *completionAssistProvider() final;
+    CppCompletionAssistProvider *functionHintAssistProvider() override;
     TextEditor::BaseHoverHandler *createHoverHandler() final;
     BaseEditorDocumentProcessor *createEditorDocumentProcessor(
             TextEditor::TextDocument *baseTextDocument) final;