c++, openmp: Handle some OMP_*/OACC_* constructs during constant expression evaluatio...

c++, openmp: Handle some OMP_*/OACC_* constructs during constant expression evaluation [PR108607]

While potential_constant_expression_1 handled most of OMP_* codes (by saying that
they aren't potential constant expressions), OMP_SCOPE was missing in that list.
I've also added OMP_SCAN, though that is less important (similarly to OMP_SECTION
it ought to appear solely inside of OMP_{FOR,SIMD} resp. OMP_SECTIONS).
As the testcase shows, it isn't enough, potential_constant_expression_1
can catch only some cases, as soon as one uses switch or ifs where at least
one of the possible paths could be constant expression, we can run into the
same codes during cxx_eval_constant_expression, so this patch handles those
there as well.

2023-02-01  Jakub Jelinek  <jakub@redhat.com>

	PR c++/108607
	* constexpr.cc (cxx_eval_constant_expression): Handle OMP_*
	and OACC_* constructs as non-constant.
	(potential_constant_expression_1): Handle OMP_SCAN and OMP_SCOPE.

	* g++.dg/gomp/pr108607.C: New test.