[Sema] Diagnose array access preceding the array bounds even when the base type is incomplete.

When the type of the base expression after IgnoreParenCasts is incomplete,
it is still possible to diagnose an array access which precedes the array
bounds.

This is a follow-up on D55862 which added an early return when the type of
the base expression after IgnoreParenCasts was incomplete.

Differential Revision: https://reviews.llvm.org/D56050

Reviewed By: efriedma

llvm-svn: 350622