openmp: Add testcase for threadprivate random access class iterators

openmp: Add testcase for threadprivate random access class iterators

This adds a testcase for random access class iterators.  The diagnostics
can be different between templates and non-templates, as for some
threadprivate vars finish_id_expression replaces them with call to their
corresponding wrapper, but I think it is not that big deal, we reject
it in either case.

2021-11-02  Jakub Jelinek  <jakub@redhat.com>

	* g++.dg/gomp/loop-8.C: New test.

(cherry picked from commit fb7fee84813b23487baf0c1094860251229ab5dd)