swift234

[SILGen] Use _ObjectiveCBridgeable._bridgeToObjectiveC for String -> NSString bridging.

Provide a general mechanism for bridging from a Swift value type to
its corresponding Objective-C class type through the
_bridgeToObjectiveC witness of the appropriate _ObjectiveCBridgeable
protocol conformance. Only enable this new code for bridging String ->
NSString and work through the issues that crop up.

We cannot actually *delete* the _convertStringtoNSString entrypoint
yet, because there is some code that is depending on it indirectly;
I'll address that separately as part of the continued generalization
of the _ObjectiveCBridgeable mechanism.