swift234

Update pass manager execution strategy for function passes.

Allow function passes to:

1. Add new functions, to be optimized before continuing with the current
   function.
2. Restart the pipeline on the current function after the current pass
   completes.

This makes it possible to fully optimize callees that are the result of
specialization prior to generating interprocedural information or making
inlining choices about these callees.

It also allows us to solve a phase-ordering issue we have with generic
specialization, devirtualization, and inlining, by rescheduling the
current function after changes happen in one of these passes as opposed
to running all of these as part of the inlining pass as happens today.

Currently this is NFC since we have no passes that use this
functionality.