Fix which Darwin versions have ObjC runtime with full subscripting support.

Update min deployment target in some tests so that they don't try
to link against libarclite and don't fail when it's not available.

rdar://problem/29253617

Reviewers: vsk, kubamracek

Reviewed By: vsk

Subscribers: jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D43787

llvm-svn: 326145