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

This reverts commit 4e9f32aa8def67e57093fe7d55ab128af768176e.

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.

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation