[Sema][ObjC] Allow declaring ObjC pointer members with non-trivial

ownership qualifications in C++ unions under ARC.

An ObjC pointer member with non-trivial ownership qualifications causes
all of the defaulted special functions of the enclosing union to be
defined as deleted, except when the member has an in-class initializer,
the default constructor isn't defined as deleted.

rdar://problem/34213306

Differential Revision: https://reviews.llvm.org/D57438

llvm-svn: 352949