demangler: Reorganize for module demangling

Module demangling requires some changes in how substitutions are
handled.  This adjusts things to make that possible.

	libiberty/
	* cp-demangle.c (d_name): Add SUBSTABLE parameter,
	push substitution if requested. Adjust unscoped name handling.
	(d_prefix): Reorder main loop. Adjust all calls.
	(d_unqualified_name): Add SCOPE parameter, create qualified
	name here. Adjust all calls.
	(cplus_demangle_type): Do not handle 'S' here, leave all
	to d_class_enum_type.
	(d_class_enum_type): Add SUBSTABLE parameter.