Handle variadic generic tuples (both concretely and in abstraction

patterns) in argument positions in reabstraction thunks.

Most of the difficulty in this work continues to center around
(1) trying to reuse as much code as possible between the parameter
and result paths and (2) propagating ownership information as
necessary throughout the code.  I did my best to assert the preconditions
and postconditions here, but undoubtedly I'm missing cases.  Some
simplicity here is still eluding me here.

This patch necessarily changes quite a bit of the code used in
non-variadic paths.  I tried to avoid doing things that I knew would
be risky, like optimizing copies.  I did fail in a few places: e.g.
we should now generate significantly better code when erasing to
Optional<Any>, just because the code was oddly poorly-factored before.
You can see the effect on the function_conversion test case.

Author: rjmccall