[dwarf-reader] const-ify Dwarf_Die* use in many places

[dwarf-reader] const-ify Dwarf_Die* use in many places

This is useful to prepare a subsequent patch that uses
get_die_pretty_type_representation with a const Dwarf_Die*.  Trying to
const-ify the use of Dwarf_Die* in that function leads to a cascade of
const-ification.  Much needed anyway.

	* src/abg-dwarf-reader.cc (get_parent_die, get_scope_die)
	(die_is_anonymous, die_is_type, die_is_decl, die_is_namespace)
	(die_is_pointer_type, pointer_or_qual_die_of_anonymous_class_type)
	(die_is_reference_type, die_is_pointer_or_reference_type)
	(die_is_qualified_type, die_has_object_pointer)
	(die_is_at_class_scope, die_unsigned_constant_attribute)
	(die_signed_constant_attribute, die_attribute_is_signed)
	(die_attribute_is_unsigned, die_attribute_has_no_signedness)
	(die_name, die_location, die_qualified_type_name)
	(die_qualified_decl_name, die_qualified_name)
	(die_qualified_type_name_empty)
	(die_return_and_parm_names_from_fn_type_die)
	(die_function_signature, die_function_type_is_method_type)
	(die_pretty_print_type, die_pretty_print_decl, die_pretty_print)
	(maybe_canonicalize_type, build_subrange_type)
	(build_subranges_from_array_type_die, compare_dies)
	(read_context::get_container)
	(read_context::compute_canonical_die_offset)
	(read_context::get_or_compute_canonical_die)
	(read_context::get_die_source)
	(read_context::get_die_qualified_type_name)
	(read_context::get_die_pretty_representation)
	(read_context::get_die_language, read_context::odr_is_relevant)
	(read_context::set_canonical_die_offset)
	(read_context::associate_die_to_type, die_is_anonymous)
	(die_string_attribute, die_constant_attribute)
	(die_attribute_has_form, die_linkage_name)
	(die_decl_file_attribute, die_die_attribute, die_size_in_bits)
	(die_is_decl, die_is_namespace)
	(pointer_or_qual_die_of_anonymous_class_type, die_is_array_type)
	(die_is_pointer_reference_or_typedef_type)
	(die_peel_pointer_and_typedef, die_function_type_is_method_type)
	(die_virtuality, die_is_virtual)
	(compare_dies_string_attribute_value, compare_dies_cu_decl_file)
	(die_location_expr, die_member_offset)
	(get_internal_anonynous_die_base_name, compare_as_decl_dies)
	(compare_as_type_dies): Const-ify the Dwarf_Die* parameter(s) of
	these functions.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>