[committed] OpenMP: Add no_openmp_constructs; improve Fortran clause parsing

[Tobias Burnus]

The 'no_openmp_constructs' assumption clause came up in a
potential performance improvement discussion and it makes
sense to permit users as early as possible to add such hints,
which can help the compiler. (For now, most assumption clauses
are ignored, except for 'holds' in an executable context; but
more might get used in the future.)

When doing the trivial change, I got odd error messages due
to partial matches of no_openmp... stating that I had
duplicated 'no_openmp' - which was purely due to partial
matches. This is now much improved (for free-form Fortran
at least) by printing a proper diagnostic and always requiring
some separator; before, in some cases it was missed. Additionally,
it avoids manual handling in most cases. [Care has been taken
such that fixed-form Fortran is not broken, albeit it is less
tested.]

Committed the attached patch as r16-6250-g4d6a43726148db.

Tobias
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