Change storage of demangle_component

This changes demangle_component objects to be stored on the obstack
that is part of demangle_info.  It also arranges for a demangle_info
object to be kept alive by cp_merge_demangle_parse_infos.  This way,
other data on the obstack can be kept while an "outer" demangle_info
needs it.

Acked-By: John Baldwin <jhb@FreeBSD.org>