c++: DECL_BUILTIN_P for builtins

We currently detect builtin decls via DECL_ARTIFICIAL &&
!DECL_HIDDEN_FUNCTION_P, which, besides being clunky, is a problem as
hiddenness is a property of the symbol table -- not the decl being
hidden.  This adds DECL_BUILTIN_P, which just looks at the
SOURCE_LOCATION -- we have a magic one for builtins.

One of the consequential changes is to make function-scope omp udrs
have function context (needed because otherwise duplicate-decls thinks
the types don't match at the point we check).  This is also morally
better, because that's what they are -- nested functions, stop lying.

(That's actually my plan for all DECL_LOCAL_DECL_P decls, as they are
distinct decls to the namespace-scope decl they alias.)

	gcc/cp/
	* cp-tree.h (DECL_BUILTIN_P): New.
	* decl.c (duplicate_decls): Use it.  Do not treat omp-udr as a
	builtin.
	* name-lookup.c (anticipated_builtin): Use it.
	(set_decl_context_in_fn): Function-scope OMP UDRs have function context.
	(do_nonmember_using_decl): Use DECL_BUILTIN_P.
	* parser.c (cp_parser_omp_declare_reduction): Function-scope OMP
	UDRs have function context.  Assert we never find a valid duplicate.
	* pt.c (tsubst_expr): Function-scope OMP UDRs have function context.
	libcc1/
	* libcp1plugin.cc (supplement_binding): Use DECL_BULTIN_P.