[flang] 2.8.1 SIMD structural checks (flang-compiler/f18#554)

Straightforward implementation but not including:
* list-item on aligned clause
* nesting check with the ordered construct (parse tree changes needed!)
* list-item attributes check on aligned clause

Original-commit: flang-compiler/f18@cf04d8ad26fc392822335a529430afb65421d7b4
Reviewed-on: https://github.com/flang-compiler/f18/pull/554