Improve looking up help ID candidates in case of code issues

This is a follow-up task of QTCREATORBUG-15959 and many related reports.
The code model fails to provide useful information for context help, even if in the same situation code completion and the global index work fine.

Examples ( | denoting cursor position when pressing F1):

QString::n|umber

tr("%1").ar|g(

Since Qt Creator 4.9 we have a text based fallback which provides the user at least with some context help, but it would be better if the code model provided better information to begin with in these situations.