[flang] Check bounds on pointer assignment

Perform checks on bounds-spec and bounds-remapping in a pointer
assignment statement:
- check that the rank of the bounds specified matches the rank of the
  pointer
- for bounds-spec, check that the pointer rank matches the target rank
- for bounds-remapping:
  - check that the target is rank 1 or simply contiguous
  - check that there are sufficient elements on the RHS for the bounds
    specified, when it can be determined at compile time

Move more of the pointer-specific checking from `assignment.cc`
to `pointer-assignment.cc`.

Original-commit: flang-compiler/f18@7489b3539224f8ad7a55873916e5854510236218
Reviewed-on: https://github.com/flang-compiler/f18/pull/944