[flang] [OpenMP] parse tree changes for `OpenMPBlockConstruct` (flang-compiler/f18#632)

* [OpenMP] parse tree changes for `OpenMPBlockConstruct`

1. merge `Workshare` and `Single` into `OpenMPBlockConstruct` because
   they both accept structured-block and syntax is similar to other block
   directives.

2. `OpenMPBlockConstruct` changes to structure like `{Begin, Block, End}`,
   where `Begin` and `End` are tuple of `{Directive, ClauseList}`.

3. Updated the check-omp-structure.* for necessary parts. Added all the END
   directive enumeration types that may have clauses.

More tests will be added during Semantics.

* [OpenMP] Update on Tim's suggestion

1. Fix unspecified enumeration for `OmpDirective` in the `OmpContext`.
   This is through getting rid of `PushContext(source)` function to
   make sure whenever it is about to push a NEW context, directive
   source location and enumeration are available. To do that, I moved
   around all the switches for directive into high level `Construct`'s
   `Enter` node. Besides fixing the issue, the side benefit is that
   whenever we call `GetContext().directive`, we are sure that the
   `directive` here was set already.

2. When `Enter` the `OmpEndBlockDirective` node, partial context
   information, such as directive source location or legal clause lists,
   needs to be reset. The new directive source location should be
   `OmpEndBlockDirective`'s `source`. The enumeration `directive`
   should not be reset for the END directives that do not accept
   clauses because nothing needs to be checked (for example any clause
   that is on `END PARALLEL` is illegal).

Original-commit: flang-compiler/f18@e5bd6b7ba0fbe9006f3e431260428b194f2d2616
Reviewed-on: https://github.com/flang-compiler/f18/pull/632