[flang] [OpenMP] parse tree changes for `OpenMPLoopConstruct` (flang-compiler/f18#656)

1. Following Block and Sections constructs, re-structure loop related
   constructs into `{Begin, Loop, End}`. Being part of the work in
   PR flang-compiler/f18#599, the `Loop` and `End` nodes are optional during parser. They
   should be filled in during the phase of `CanonicalizationOfOmp`. This
   commit is solely for the parse tree change. So, after this commit,
   PR flang-compiler/f18#599 needs to be changed accordingly.

2. Removed parse tree nodes for `END DO` and `END DO SIMD`. Similar to
   Block and Sections constructs, `End` node now accepts clauses too,
   the validity checks are deferred into Semantics. This is more genernal
   and error message could be better.

3. With this commit alone, assertion error would occur when `End` directive
   is present, for example `!$OMP END DO` because the `End` node is not
   moved into `OpenMPLoopConstruct` yet. Again, PR flang-compiler/f18#599 will handle that.

More tests will be added in PR flang-compiler/f18#599 and during the future Semantics work.

Original-commit: flang-compiler/f18@8cd1932fd61fa67f8ad5abfd337cf7a223ea89f4
Reviewed-on: https://github.com/flang-compiler/f18/pull/656