[Patch] OpenMP: Add 'omp requires' to Fortran (mostly parsing)

[Tobias Burnus]

Attached is an updated version – whether that will be fully
in line with OpenMP 5.1 remains to be seen. But in any case,
it now handles properly:
"If a directive appears in the declarative part of a module then the
behavior is as if that directive appears after any references to that
module."

Note: That with the current implementation, the restriction that
the 'requires' cannot come after an 'omp atomic' cannot occur as
'omp requires' belongs into the specification part of a 'program unit'
(Fortran; OpenMP: 'compilation unit') and 'omp atomic' belongs to
the execution part.
Thus, unless a later 'use m' implicitly provides the clause, it
cannot occur. (The current implementation does not permit that
use-stmt which is not at 'program unit' level can introduce new
clauses applicable at 'program unit' level.)

That's different to target as an 'omp declare target' also
belongs into the specification section. (Additionally, that
checking variable is also used for ensure that all program units
use the same offload clauses of omp requires.)

There is no 'use m' test for the three offload-related items
as the 'sorry, not implemented' prevents the creation of the
module file.

OK?

Tobias

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