swift234

Enable cast optimizations as a guaranteed optimization.

Fix a few places in diagnose unreachable, which were emitting false diagnostics after enabling this optimization.

This commit does not disable and remove the special-casing for bridging conversions in the CSApply yet. Let's wait a bit and see that everything works properly after my recent cast optimizations changes. If everything is OK, we can remove the code from CSApply.

Swift SVN r27379