swift234

Really reinstate "[SILGen] Implement NSString -> String bridging through _ObjectiveCBridgeable."

This reverts commit 052d2d0a6906b2d9b89ec0e7f481a1e6992ef098.

The only actual issue with the original change was a missing change to
the UIApplicationMain SILGen test, which needs to build SILGen
overlays to execute properly; -enable-source-import doesn't suffice.