(Patch + RFC.)

OpenMP 5.2 has 'ompx' and (for fixed source form Fortran) 'omx'
as sentinel to provide a defined namespace for vendor extensions.

The behavior when encountering an unknown directive with ompx/omp sentinel
(or an unknown clause with ompx_ prefix) is implementation defined. For unknown
clauses there will be always an error in GCC.
For sentinels, GCC compiles the code and ignores the directive.
From the user perspective, either silently ignoring the directive or
showing a warning or showing an erroring is best - depending on the semantic
of that vendor extension. As the semantic is not known, providing a means
to warn makes most sense.

For warning, we can either show a
- specific message for ompx/omx - or a
- generic message
And we can either use some existing generic flag (unknown pragmas,
attributes, suprising behavior etc.), depending on language and source
language (C, C++, Fortran fixed/free)- or add & use a specific -W flag.

The attached patch just documents the existing warning behavior, which
is quite diverse + adds testcases for them. As fixed-form Fortran had no
warning, a new warning with a specific message was added.
(Cf. testcases in the patch for what's shown as message.)

OK for mainline - or other ideas how to handle this topic best?

Tobias


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