Don't emit `-Wnullability-completeness` warnings on `weak` Objective-C properties.

Zeroing weak references are by definition `nullable`, and adding
`nonnull` or `_Nullable` yields a mutual-exclusivity error. When
`-Wnullability-completeness` is enabled, however, non-audited header
regions require adding the `nullable` property keyword to avoid a
warning. This should be unnecessary, since it restates known nullability
of the `weak` property.

Additionally, the fix-it hints are both non-idiomatic Objective-C
(adding `_Nullable` to the property's pointer type rather than in the
`@property` attributes) and suggest the option of adding `_Nonnull`,
which would be an error.

Differential Revision: https://reviews.llvm.org/D128031