OpenMPBlockConstruct, somewhat confusingly, represents most but not all
block-associated constructs. It's derived from OmpBlockConstruct, as are
all the remaining block-associated constructs.

It does not correspond to any well-defined group of constructs. It's the
collection of constructs that don't have their own types (and those that
do have their own types do so for their own reasons).

Using the broader OmpBlockConstruct in type-based visitors won't cause
issues, because the specific overloads (for classes derived from it)
will always be preferred.

This allows not having the END CRITICAL directive in certain situations.
Update semantic checks and symbol resolution.

The structure is
- OmpBeginDirective (aka OmpDirectiveSpecification)
- Block
- optional<OmpEndDirective> (aka optional<OmpDirectiveSpecification>)

The OmpBeginDirective and OmpEndDirective are effectively different
names for OmpDirectiveSpecification. They exist to allow the semantic
analyses to distinguish between the beginning and the ending of a block
construct without maintaining additional context.

The actual changes are in the parser: parse-tree.h and openmp-parser.cpp
in particular. The rest is simply changing the way the directive/clause
information is accessed (typically for the simpler).

All standalone and block constructs now use OmpDirectiveSpecification to
store the directive/clause information.

Create these new files in flang/lib/Semantics:
  openmp-utils.cpp/.h         - Common utilities
  check-omp-atomic.cpp        - Atomic-related checks
  check-omp-loop.cpp          - Loop constructs/clauses
  check-omp-metadirective.cpp - Metadirective-related checks

Update lists of included headers, std in particular.

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Co-authored-by: Jack Styles <jack.styles@arm.com>