swift234

Allow overrides to leave out the ownership qualifier of the decl they override.

If they do provide it, however, it must match. So the following combinations
are valid:

  weak var -> override weak var
  weak var -> override var

but these are not:

  var -> override weak var
  weak var -> override unowned var

To be fully correct, we should be cloning the attribute down and adjusting
the type of the overriding property, but I'd rather not touch that right now.

<rdar://problem/17837100>

Swift SVN r20694