[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