demangle.h (qualifier_list<Allocator>::decode_qualifiers(string_type&, string_type&, bool) const): Made const.

* include/bits/demangle.h
(qualifier_list<Allocator>::decode_qualifiers(string_type&,
string_type&, bool) const): Made const.
(qualifier_list<Allocator>::M_printing_suppressed): Added mutable.
(_GLIBCXX_DEMANGLER_DOUT_ENTERING3, _GLIBCXX_DEMANGLER_RETURN3,
std::ostream& operator<<(std::ostream&, qualifier const&),
std::ostream& operator<<(std::ostream&, qualifier_list const&),
qualifier_list<Allocator>::decode_qualifiers(string_type&,
string_type&, bool) const,
session<Allocator>::decode_type_with_postfix(string_type&,
string_type&, qualifier_list<Allocator>*))
Added and/or changed debug-only hooks and code.

From-SVN: r73327