Fix inference of _Nullable for weak Objective-C properties.

The inference of _Nullable for weak Objective-C properties was broken
in several ways:

* It was back-patching the type information very late in the process
  of checking the attributes for an Objective-C property, which is
  just wrong.
* It was using ad hoc checks to try to suppress the warning about
  missing nullability specifiers (-Wnullability-completeness), which
  didn't actual work in all cases (rdar://problem/22985457)
* It was inferring _Nullable even outside of assumes-nonnull regions,
  which is wrong.

Putting the inference of _Nullable for weak Objective-C properties in
the same place as all of the other inference logic fixes all of these
ills.

llvm-svn: 249896