[flang][OpenMP] Parse strictly- and loosely-structured blocks (#150298)

Block-associated constructs have, as their body, either a strictly- or a
loosely-structured block. In the former case the end-directive is
optional.

The existing parser required the end-directive to be present in all
cases.

Note:
The definitions of these blocks in the OpenMP spec exclude cases where
the block contains more than one construct, and the first one is
BLOCK/ENDBLOCK. For example, the following is invalid:
```
  !$omp target
  block         ! This cannot be a strictly-structured block, but
    continue    ! a loosely-structured block cannot start with
  endblock      ! BLOCK/ENDBLOCK
  continue      !
  !$omp end target
```

0 files changed, 0 insertions, 0 deletions