[flang] Add checks for valid defined assignment procedures

Perform the checks from 15.4.3.4.3 to determine what procedures are
valid to implement defined assignment. This requires characterizing
procedures, so share the result of that with
`CheckSpecificsAreDistinguishable`.

Original-commit: flang-compiler/f18@9e0d79f1735cf99b43f9ff7f6d712d7aa862dead
Reviewed-on: https://github.com/flang-compiler/f18/pull/841
Tree-same-pre-rewrite: false