swift234

SIL: Start cleaning up getMethodDispatch() / requiresObjCDispatch()

Move these to SILDeclRef, maybe not the best place but a good home for now.
Factor out a new requiresForeignToNativeThunk() function, which cleans up
some code duplication introduced by the following patch:

https://github.com/apple/swift/commit/478e1c7513cef39548d0579979a68bf260b97975

This is a small step towards consolidating duplicated logic for figuring out
method dispatch semantics and emitting curry thunks.