swift234

Improve property overriding semantics.

A few improvements in our checking of property overrides:
  * Properly check for an extraneous @override on a property
  * Don't allow overriding of stored properties or with a stored property
  * Only allow a covariant override when the overridden property is not mutable
  * Check overrides as part of validation, not in a post-pass

Swift SVN r14403