swift234

SILGen: Emit @objc thunks with external linkage, even when local.

We do this hack for @objc methods at the top level, emitting them as external in SIL and making them private in IRGen, to prevent SIL optimizations from dropping the functions as unused (see <rdar://problem/17074598>), but we weren't doing so for @objc classes in local contexts, and they were getting dropped. Fixes <rdar://problem/16982281>.

Swift SVN r18668