[analyzer] Use getParentIgnoreParenCasts instead of doing it by hand.

Apart from being more compact and already implemented, this also handles the
case where the parent is null. (It does also ignore all casts, not just
implicit ones, but this is more efficient to test and in the case we care
about---a message in a PseudoObjectExpr---there should only be implicit casts
anyway.

This should fix our internal buildbot.