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/clangcompletionassistprocessor.cpp

@@ -347,6 +347,7 @@ IAssistProposal *ClangCompletionAssistProcessor::startCompletionHelper()
                                               analyzer.functionNameStart());
         break;
     }
+    case ClangCompletionContextAnalyzer::CompleteNone:
     default:
         break;
     }