swift234

Library evolution tests: add test for changing body of a transparent function

I'm going to add some more tests related to @_transparent shortly,
and it is important to verify that bodies of @_transparent functions
are serialized (at least in single-frontend mode, for now).

I realize this test is probably too specific, since in the future
we may sometimes choose not to inline @_transparent functions into
their callers, even if a serialized body is available; but this
will suffice for now.