This is the first proof of concept of the modification of FIR lowering
to fully support a variety of top level operations (beyond just
func.func) proposed in
https://discourse.llvm.org/t/rfc-add-an-interface-for-top-level-container-operations

One unfortunate side-effect of this is that the new AbstractResult pass
cannot be scheduled on a builtin.module operation and so we can't use
  fir-opt --abstract-result < file.fir

I tried adding support for operating on a module to the pass, but this
wasn't straightforward. Operating at module scope means that conversions
added for return operations run on every return operation in the module
rather than just in the current function and this violates assumptions
in the pass: producing incorrect results. This doesn't effect normal
operation because the pass manager will always run the pass on a
specific top level operation not on a whole module. I have worked around
this by specifying the pass pipeline more specifically in the tests.

I expect most other passes will be able to keep their old fir-opt
interface.

Co-authored-by: Markus Böck <markus.boeck02@gmail.com>