cleanup: constify "struct attribute" function parameter

cleanup: constify "struct attribute" function parameter

gdb/

2013-07-30  Sanimir Agovic  <sanimir.agovic@intel.com>

	* dwarf2read.c (dwarf2_get_ref_die_offset): Constify struct
	attribute parameter.
	(dwarf2_const_value_data): Constify struct attribute parameter.
	(dwarf2_const_value): Constify struct attribute parameter.
	(dwarf2_const_value_attr): Constify struct attribute parameter.
	(lookup_die_type): Constify struct attribute parameter.
	(dwarf2_get_attr_constant_value): Constify struct attribute parameter.
	(follow_die_ref_or_sig): Constify struct attribute parameter.
	(follow_die_ref): Constify struct attribute parameter.
	(follow_die_sig): Constify struct attribute parameter.
	(get_DW_AT_signature_type): Constify struct attribute parameter.
	(get_type_unit_group): Constify struct attribute parameter.
	(fill_in_loclist_baton): Constify struct attribute parameter.
	(dwarf2_symbol_mark_computed): Constify struct attribute parameter.
	(type_unit_group): Constify struct attribute parameter.