swift234

Sema: Recommit "Disallow potential unavailability on stored properties" without Fix-It

For now, disallow potential unavailability on stored properties. We will
eventually want to support this. Unfortunately, doing so will require changes to
definite initialization and (probably) deinitialization.

This is the same as the reverted r27216 except that it does not suggest a FixIt
to make the property lazy.

Swift SVN r27223