swift234

Two modifications to the SIL linker:

- First, it now knows how to look through
  PartialApplyInsts in addition to simple
  ApplyInsts when finding functions to link.

- Second, it now consults the external source
  in all cases where CalleeFunction is non-NULL
  rather than just if the CalleeFunction is
  non-NULL and empty.  This allows the external
  source to override functions with pre-existing
  bodies.


Swift SVN r9836