[flang] Batch together the changes to the PFT intermediate data

structure for upstreaming to llvm-project.

These files have had many changes since they were originally upstreamed.
Some of the changes are cosmetic.  Most of the functional changes were
done to support the lowering of control-flow syntax from the front-end
parse trees to the FIR dialect.

This patch is meant to be a reviewable size. The functionality it
provides will be used by code yet to be upstreamed in lowering.

review comments:

[review D80449][NFC] make PFT ParentVariant a ReferenceVariant

ReferenceVariant had to be slightly updated to also support
non constant references (which is required for ParentType).

[review D80449] extend Variable implementation beyond a comment