...and maybe other cases.

Since

    Clang: fix findStartOfName handling
    commit 82d0650b



the proposal's base position was calculated wrong. As a result, an early
return triggert in CodeAssistantPrivate::displayProposal (call to
newProposal->hasItemsToPropose(prefix, reason)) and no completions were
displayed.

Fix by ensuring that the added code from the mentioned commit is only
called when needed, namely only for function expressions.

Task-number: QTCREATORBUG-19083
Change-Id: I8f23c9b7186f9d81159939c8b3ef475a09bbe760
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>