[Patch] Fortran/OpenMP: Fix strictly structured blocks parsing

[Tobias Burnus]

This patch is about error diagnostic + an ICE for invalid code.

Before the patch, gfortran/f951 showed:

...
Error: 'ancestor' device modifier not preceded by 'requires' directive with 'reverse_offload' clause
   18 | end block
      |   1
Error: Expecting END PROGRAM statement at (1)
gfortran: internal compiler error: Segmentation fault signal terminated program f951

* The first error is for a user error and fine.
* A follow-up error is expected (due to the now dangling '!$omp end target'),
  but the error location and wording is misleading
* And the ICE is plainly wrong.


With the patch, there is no ICE and the the second error reads:

   16 |   !$omp end target
Error: Unexpected !$OMP END TARGET statement at (1)


The problem was due to nesting '!$omp target' (= same directive),
where the outer one was a strictly structured block - and parsing
the inner '!$omp target' failed with MATCH_ERROR, which ignored that
line - while '!$omp end target' remained.

(So far, so good, but then the parsing-code did run into a bug.)

For the blocks, the following applies. OpenMP permits either
* strictly structured blocks (with optional END_ST == 'end target')
  !$omp target
    block
      ...
    end block
  !$omp end target  ! << this line is optional
* loosely structured block
  !$omp target
     ... ! may not start with 'block' (and hence cannot end with 'end block')
  !$omp end target  ! << required


The parsing issue is in the following code,
which first takes care of the 'strictly':  'end block' + optional 'end target'
and then of the 'loosely structured' case with just:  'end target':

      else if (block_construct && st == ST_END_BLOCK)
        ...
          st = next_statement ();
          if (st == omp_end_st)
              accept_statement (st);
        ...
      else if (st != omp_end_st)
        {
          unexpected_statement (st);
          st = next_statement ();
        }

The fix is to change the second if condition to:
      else if (st != omp_end_st || (block_construct && st == omp_end_st))

or rather to the following equivalent code:
      else if (st != omp_end_st || block_construct)


OK for mainline and GCC 12?*

Tobias

*strictly structured blocks were added in r12-4592.


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