swift234

[local] Add tryDeleteDeadClosures.

This utility attempts to delete dead closures with a set of
post-dominating releases using the infrastructure from
getFinalReleasesForValue.

It currently only will eliminate closures that only have retain, release
uses and a final post-dominating release set.

The reason why we need the final post-dominating release set is so that
we can release any captured variables at the points where we would have
deallocated the release. This is b/c captured variables are passed in at
+1 to partial apply.

Swift SVN r25050