[flang] [OpenMP] Add Sections and Single Construct check (flang-compiler/f18#585)

* [OpenMP] Add Sections and Single Construct check

Parse tree for OmpEndSingle needs to be modified to save the provenance
of END SINGLE directive and check its own clauses

* Update on reviews

1. PushContext is created to push new context with source provenance

2. Tweak the logic for SECTION nesting, treak Orphaned or wrong nesting
   as the same error type

3. Make sure the check for NOWAIT clause only applies to the ones that
   are not handled by parser.
   Note that the case for DO or DO_SIMD will take effect after the
   loop association work (parse tree change) is done. But I still list
   them there for completeness.

4. Happen to find that NOWAIT is not accepted by PARALLEL SECTIONS,
   fixed it in the parser.


Original-commit: flang-compiler/f18@236cf1efea77b8af5c533aa55029ce49a2c2e72c
Reviewed-on: https://github.com/flang-compiler/f18/pull/585