swift234

[SE-0125] Remove `isUniquelyReferenced` and the `NonObjectiveCBase` class

We can express the same using the `isUniquelyReferencedNonObjC` API.

- Rename `isUniquelyReferencedNonObjC` to `isKnownUniquelyReferenced`.
- Cleanup `ManagedBufferPointer` by removing holdsUniqueOrPinnedReference` and
  renaming `holdsUniqueReference` to `isUniqueReference`.
- No longer promise to return false from `isKnownUniquelyReferenced` for @objc
  class instances.

SR-1962
rdar://21886410