swift234

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

Introduce a new entrypoint to _ObjectiveCBridgeable,
_unconditionallyBridgeFromObjectiveC, which handles unconditional
bridging from an optional Objective-C object (e.g., an NSString) to
its bridged Swift type. Use it in SILGen to perform NSString -> String
bridging rather than the custom entry point.

Another small step toward generalized bridging.