Fix PR c++/60573

	PR c++/60573
	* name-lookup.h (cp_binding_level): New transient field defining_class_p
	to indicate whether a scope is in the process of defining a class.
	* semantics.c (begin_class_definition): Set defining_class_p.
	* name-lookup.c (leave_scope): Reset defining_class_p.
	* parser.c (synthesize_implicit_template_parm): Use cp_binding_level::
	defining_class_p rather than TYPE_BEING_DEFINED as the predicate for
	unwinding to class-defining scope to handle the erroneous definition of
	a generic function of an arbitrarily nested class within an enclosing
	class.

	PR c++/60573
	* g++.dg/cpp1y/pr60573.C: New testcase.

From-SVN: r208921