I intent to commit the attached patch soon.

However, I want to give anyone the chance to comment on any aspect before
committing. Comments after the commit are welcome as well :-)

OpenMP 5.2 now uses properties to clauses and "pure" is among those properties.

Note that pure-2.f90 contains also stubs for directives only added in TR11 or TR12
to reduce the chance of missing those once they get implemented.
Additionally, 'scan' is 'pure' only since very recently - which I read
as bug fix; hence, it is accepted with the attached patch.

Tobias
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