openmp: Don't try to destruct DECL_OMP_PRIVATIZED_MEMBER vars [PR108180]

DECL_OMP_PRIVATIZED_MEMBER vars are artificial vars with DECL_VALUE_EXPR
of this->field used just during gimplification and omp lowering/expansion
to privatize individual fields in methods when needed.
As the following testcase shows, when not in templates, they were handled
right, but in templates we actually called cp_finish_decl on them and
that can result in their destruction, which is obviously undesirable,
we should only destruct the privatized copies of them created in omp
lowering.

Fixed thusly.

2022-12-21  Jakub Jelinek  <jakub@redhat.com>

	PR c++/108180
	* pt.cc (tsubst_expr): Don't call cp_finish_decl on
	DECL_OMP_PRIVATIZED_MEMBER vars.

	* testsuite/libgomp.c++/pr108180.C: New test.