[flang] Add more checks on WHERE and FORALL

[flang] Add more checks on WHERE and FORALL

Check that masks and LHS of assignments in WHERE statements and
constructs have consistent shapes. They must all have the same rank and
any extents that are compile-time constants must match.

Emit a warning for assignments in FORALL statements and constructs where
the LHS does not reference each of the index variables.

Original-commit: flang-compiler/f18@8b04dbebcf5621cfd571a8c45878cebcd1a1bfb0
Reviewed-on: https://github.com/flang-compiler/f18/pull/1009