openmp: Ensure copy ctor for composite distribute parallel for class iterators is instantiated [PR95197]

During gimplification omp_finish_clause langhook is called in several places
to add the language specific info to the clause like what default/copy ctors,
dtors and assignment operators should be used.

Unfortunately, if it refers to some not yet instantiated method, during
gimplification it is too late and the methods will not be instantiated
anymore.  For other cases, the genericizer has code to detect those and
instantiate whatever is needed, this change adds the same for
distribute parallel for class iterators where we under the hood need
a copy constructor for the iterator to implement it.

2020-05-26  Jakub Jelinek  <jakub@redhat.com>

	PR c++/95197
	* gimplify.c (find_combined_omp_for): Move to omp-general.c.
	* omp-general.h (find_combined_omp_for): Declare.
	* omp-general.c: Include tree-iterator.h.
	(find_combined_omp_for): New function, moved from gimplify.c.

	* cp-gimplify.c: Include omp-general.h.
	(cp_genericize_r) <case OMP_DISTRIBUTE>: For class iteration
	variables in composite distribute parallel for, instantiate copy
	ctor of their types.

(cherry picked from commit f1f862aec2c3b93dbd6adfc35b0e1b6034e59c21)