While looking at PR96184 I noticed that we were recognizing the situation of
parsing a function declarator based on current_binding_level, and that we
ought to make that a predicate function.  This patch is just refactoring,
but I just suggested using it in a review of another patch.

gcc/cp/ChangeLog:

	* cp-tree.h (parsing_function_declarator): Declare.
	* name-lookup.c (set_decl_context_in_fn): Use it.
	* parser.c (cp_parser_direct_declarator): Use it.
	(parsing_function_declarator): New.