swift234

[SILGen] Eliminate special-case bridging code for _ObjectiveCBridgeable types.

SILGen's bridging logic was recognizing the various bridged types
(String/Array/Dictionary/Set) using ad-hoc logic, then jumping into
the generic, _ObjectiveCBridgeable-based bridging logic. Replace the
ad-hoc logic with an _ObjectiveCBridgeable conformance query.

SILGen still has Array/Dictionary/Set-specific hacks for collection
up/downcasting, but the bridging code is now generic.