[analyzer] DeallocChecker: Don't warn about directly-set IBOutlet ivars on macOS

On macOS (but not iOS), if an ObjC property has no setter, the nib-loading code
for an IBOutlet is documented as directly setting the backing ivar without
retaining the value -- even if the property is 'retain'.

This resulted in false positives from the DeallocChecker for code that did not
release such ivars in -dealloc.

To avoid these false positives, treat IBOutlet ivars that back a property
without a setter as having an unknown release requirement in macOS.

rdar://problem/28507353

llvm-svn: 284084