swift234

Emit writebacks during error-handling cleanup.

This requires some really ugly copies at -O0 in order to
avoid changing whether enclosing cleanups are active during
the emission of those cleanups.  (There may also be complex
situations where values are forwarded to the writeback but
the writeback cleanup can't take advantage of that --- not
sure yet.)  These should generally be trivial to clean up,
but I'm still thinking about how to avoid emitting them in
the first place.

Swift SVN r30180