[flang] Change `ArraySpec` from a type alias to a class

This allows it to have member functions `Rank()`, `IsExplicitShape()`,
`IsAssumedShape()`, etc. Make use of those new functions and remove
`isExplicit()` and `isDeferred()` from `ShapeSpec` as they are no
longer needed.

Original-commit: flang-compiler/f18@7ef7ad63594db9c41a0889d2ad419dbdf841cb6f
Reviewed-on: https://github.com/flang-compiler/f18/pull/630
Tree-same-pre-rewrite: false