[ObjC] Allow declaring __weak pointer fields in C structs in ARC.

This patch uses the infrastructure added in r326307 for enabling
non-trivial fields to be declared in C structs to allow __weak fields in
C structs in ARC.

rdar://problem/33599681

Differential Revision: https://reviews.llvm.org/D44095

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@327206 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit a4a75f6417166ff8d689a41c26a68b9c979e8e42)