OpenMP: Add declare variant's 'append_args' clause in C/C++

OpenMP: Add declare variant's 'append_args' clause in C/C++

Add the append_args clause of 'declare variant' to C and C++,
fix/improve diagnostic for 'interop' clause and 'declare_variant'
clauses on the way.

Cleanup dispatch handling in gimplify_call_expr a bit and
partially handle 'append_args'. (Namely, those parts that
do not require libraries calls, i.e. a dispatch construct
where the 'device' and 'interop' clause has been specified.)

The sorry can be removed once an enum value like
  omp_ipr_(ompx_gnu_)omp_device_num (cf. OpenMP Spec Issue 4451)
has to be added to the runtime side such that omp_get_interop_int
returns the device number of an interop object (as passed to
dispatch via the interop clause); and a call to GOMP_interop
has to be added to create interop objects. Once available, only
a very localized change in gimplify_call_expr is required to
claim for full support. - And Fortran parsing support.

gcc/c-family/ChangeLog:

	* c-omp.cc (c_omp_interop_t_p): Handle error_mark_node.

gcc/c/ChangeLog:

	* c-parser.cc (c_parser_omp_clause_init_modifiers): New;
	split of from ...
	(c_parser_omp_clause_init): ... here; call it.
	(c_finish_omp_declare_variant): Parse 'append_args' clause.
	(c_parser_omp_clause_interop): Set tree used/read.

gcc/cp/ChangeLog:

	* decl.cc (omp_declare_variant_finalize_one): Handle
	append_args.
	* parser.cc (cp_parser_omp_clause_init_modifiers): New;
	split of from ...
	(cp_parser_omp_clause_init):  ... here; call it.
	(cp_parser_omp_all_clauses): Replace interop parsing by
	a call to ...
	(cp_parser_omp_clause_interop): ... this new function;
	set tree used/read.
	(cp_finish_omp_declare_variant): Parse 'append_args' clause.
	(cp_parser_omp_declare): Update comment.
	* pt.cc (tsubst_attribute, tsubst_omp_clauses): Handle template
	substitution also for declare variant's append_args clause,
	using for 'init' the same code as for interop's init clause.

gcc/ChangeLog:

	* gimplify.cc (gimplify_call_expr): Update for OpenMP's
	append_args; cleanup of OpenMP's dispatch clause handling.

gcc/testsuite/ChangeLog:

	* c-c++-common/gomp/declare-variant-2.c: Update dg-error msg.
	* c-c++-common/gomp/dispatch-12.c: Likewise.
	* c-c++-common/gomp/dispatch-11.c: Likewise and extend a bit.
	* c-c++-common/gomp/append-args-1.c: New test.
	* c-c++-common/gomp/append-args-2.c: New test.
	* c-c++-common/gomp/append-args-3.c: New test.
	* g++.dg/gomp/append-args-1.C: New test.
	* g++.dg/gomp/append-args-2.C: New test.
	* g++.dg/gomp/append-args-3.C: New test.