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.