c++: local-scope OMP UDR reductions have no template head

c++: local-scope OMP UDR reductions have no template head

This corrects the earlier problems with removing the template header
from local omp reductions.  And it uncovered a latent bug.  When we
tsubst such a decl, we immediately tsubst its body.
cp_check_omp_declare_reduction gets a success return value to gate
that instantiation.

udr-2.C got a further error, as the omp checking machinery doesn't
appear to turn the reduction into an error mark when failing.  I
didn't dig into that further.  udr-3.C appears to have been invalid
and accidentally worked.

	gcc/cp/
	* cp-tree.h (cp_check_omp_declare_reduction): Return bool.
	* semantics.c (cp_check_omp_declare_reduction): Return true on for
	success.
	* pt.c (push_template_decl_real): OMP reductions do not get a
	template header.
	(tsubst_function_decl): Remove special casing for local decl omp
	reductions.
	(tsubst_expr): Call instantiate_body for a local omp reduction.
	(instantiate_body): Add nested_p parm, and deal with such
	instantiations.
	(instantiate_decl): Reject FUNCTION_SCOPE entities, adjust
	instantiate_body call.
	gcc/testsuite/
	* g++.dg/gomp/udr-2.C: Add additional expected error.
	libgomp/
	* testsuite/libgomp.c++/udr-3.C: Add missing ctor.