Make sure the BackwardScanner has tokenized before asking for a string.

The problem was that for

Foo::Foo
(|)

when the user triggers completion at |, startOfLine() returns a token
that's not on the current line and not yet tokenized by that instance
of the BackwardScanner.

As a fix I explicitly ask the instance to tokenize up to the given
index.

Task-number: QTCREATORBUG-673
Reviewed-by: Roberto Raggi