OpenMP: Fix C++ template handling with append_args' prefer_type modifier

OpenMP: Fix C++ template handling with append_args' prefer_type modifier

It is possible but not very sensible to use C++ templates with in the
prefer_type modifier to the 'append_args' clause of 'declare variant'.

The commit r15-6336-g12dd892b1a3ad7 added substitution support in pt.cc,
but missed to update afterward the actual data in decl.cc.
As gimplification support was only added in r15-8898-gf016ee89955ab4,
this could not be tested back then. The latter commit added a sorry
for it gimplify.cc and the existing testcase, which this commit now removes.

gcc/cp/ChangeLog:

	* cp-tree.h (cp_finish_omp_init_prefer_type): Add.
	* decl.cc (omp_declare_variant_finalize_one): Call it.
	* pt.cc (tsubst_attribute): Minor rebustification for OpenMP
	append_args handling.
	* semantics.cc (cp_omp_init_prefer_type_update): Rename to ...
	(cp_finish_omp_init_prefer_type): ... this; remove static attribute
	and return modified tree. Move clause handling to ...
	(finish_omp_clauses): ... the caller.

gcc/ChangeLog:

	* gimplify.cc (modify_call_for_omp_dispatch): Remove sorry.

gcc/testsuite/ChangeLog:

	* g++.dg/gomp/append-args-1.C: Remove expected dg-sorry.
	* g++.dg/gomp/append-args-8.C: New test.