swift234

Explicitly represent "pseudo-generic" functions in SIL, which do
not have access to their type arguments at runtime.  Use this to
fix the emission of native thunks for imported ObjC-generic
initializers, since they may need to perform bridging.

For now, pseudo-genericity is all-or-nothing, but we may want to
make it apply only to certain type arguments.

Also, clean up some code that was using dead mangling nodes.