swift234

SILGen: Emit a single epilog block for returns.

Funnel all top-level cleanups and returns through a single epilog block instead of re-emitting the cleanups and returns for every 'return' statement like an idiot.

This has some interesting interactions with other features that need additional work:

- ReturnInst's SILLocation is no longer naturally associated with a single source location, which complicates return analysis diagnostics. For now, I just save the location of the first ReturnStmt we see and tag the ReturnInst with that, but we should do better.

- We still duplicate cleanups below the top level like chumps. I still need to resurrect the CleanupOutflows stuff from the old IRGen backend.

- A DebugInfo test breaks. Adrian will have to look into that.

Swift SVN r6782