[Flang][mlir] add a band-aid to support the creation of mutually recursive...

[Flang][mlir] add a band-aid to support the creation of mutually recursive types when lowering to LLVM IR

Summary:
This is a temporary implementation to support Flang.  The LLVM-IR parser
will need to be extended in some way to support recursive types.  The
exact approach here is still a work-in-progress.

Unfortunately, this won't pass roundtrip testing yet. Adding a comment
to the test file as a reminder.

Differential Revision: https://reviews.llvm.org/D72542