Fix an assertion failure regression in isDesignatorAtObjectEnd for

Description

Fix an assertion failure regression in isDesignatorAtObjectEnd for
__builtin_object_size with incomplete array type in struct

The commit r316245 introduced a regression that causes an assertion failure when
Clang tries to cast an IncompleteArrayType to a PointerType when evaluating
__builtin_object_size.

rdar://36094951

Differential Revision: https://reviews.llvm.org/D41405