[Sema] Don't try to account for the size of an incomplete type in CheckArrayAccess

When checking that the array access is not out-of-bounds in CheckArrayAccess
it is possible that the type of the base expression after IgnoreParenCasts is
incomplete, even though the type of the base expression before IgnoreParenCasts
is complete. In this case we have no information about whether the array access
is out-of-bounds and we should just bail-out instead. This fixes PR39746 which
was caused by trying to obtain the size of an incomplete type.

Differential Revision: https://reviews.llvm.org/D55862

Reviewed By: efriedma

llvm-svn: 349811