[fir] Add affine demotion pass

Add affine demotion pass.
Affine dialect's default lowering for loads and stores is different from
fir as it uses the `memref` type. The `memref` type is not compatible with
the Fortran runtime. Therefore, conversion of memory operations back to
`fir.load` and `fir.store` with `!fir.ref<?>` types is required.

This patch is part of the upstreaming effort from fir-dev branch.

Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: V Donaldson <vdonaldson@nvidia.com>
Co-authored-by: Rajan Walia <walrajan@gmail.com>
Co-authored-by: Sourabh Singh Tomar <SourabhSingh.Tomar@amd.com>
Co-authored-by: Valentin Clement <clementval@gmail.com>

Reviewed By: schweitz

Differential Revision: https://reviews.llvm.org/D111257