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

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@352949 91177308-0d34-0410-b5e6-96231b3b80d8