[flang][codegen] Add `fir.end` conversion placeholder

This patch extends the FIRToLLVMLowering pass in Flang by adding a hook
to transform fir.end. This is just a placeholder for now as fir.end is
not required yet.

This is part of the upstreaming effort from the `fir-dev` branch in [1].

[1] https://github.com/flang-compiler/f18-llvm-project



Patch originally written by:
Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>

Differential Revision: https://reviews.llvm.org/D113295