[flang] [OpenMP] parse tree changes for standalone directives (flang-compiler/f18#627)

* [OpenMP] parse tree changes for standalone directives

1. Put all standalone directives except FLUSH, CANCEL, and CANCELLATION POINT
   into one `OpenMPSimpleStandaloneConstruct` (for no-clause directive,
   validity checks will be deferred to Semantics). A top-level class will
   include all the standalone directive nodes. This simplies the logic a lot.

2. All the standalone directives now have their own source provenance for
   directive name itself.

3. Change check-omp-structure.* to avoid assertions

4. Add basic tests for standalone directives, more will be added during
   the clause validity checks in Semantics

* Resolve !$OMP ORDERED ambiguity by attempting block construct first - Peter

Original-commit: flang-compiler/f18@a77aa7ed84880927dea3e2b1bb6433350f99f1ac
Reviewed-on: https://github.com/flang-compiler/f18/pull/627