swift234

Use a callback in the linker to notify clients of newly deserialized functions.

Previous attempts to update the callgraph explicitly after calls to
linkFunction() weren't completely effective because we can deserialize
deeply and introduce multiple new function bodies in the process.

This gets us a bit closer, but only adds new call graph nodes. It does
not currently add edges for everything that gets deserialized (and this
is not fatal, so it is a step forward).

Swift SVN r27120