[CodeGen] Add support for IncompleteArrayType in Obj-C ivars.

Fixes an assertion failure when ivar is a struct containing incomplete
array. Also completes support for direct flexible array members.

rdar://problem/21054495

Reviewers: rjmccall, theraven

Reviewed By: rjmccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D38774

llvm-svn: 316723