[flang] Handle substring of array element in EQUIVALENCE

A substring of an array element is legal as an equivalence object,
e.g. `a(2,3)[4:5]`. Extend `EquivalenceObject` to include the start
of the substring, if any. Split `CheckBound` into `CheckArrayBound`
and `CheckSubstringBound`.

When evaluating bounds in `AddToSet` we can assert they are constant
because all of the error cases should have been detected already.

Original-commit: flang-compiler/f18@0b4050b2d56e166458c5d9259234a1d404e467e4
Reviewed-on: https://github.com/flang-compiler/f18/pull/494