Convert read_address to a method on comp_unit_head

This changes read_address to be a method on comp_unit_head.

2020-02-08  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (read_address): Move to comp-unit.c.
	(dwarf2_rnglists_process, dwarf2_ranges_process)
	(read_attribute_value, dwarf_decode_lines_1)
	(var_decode_location, decode_locdesc): Update.
	* dwarf2/comp-unit.c (comp_unit_head::read_address): Move from
	read.c.  Remove "cu" parameter.
	* dwarf2/comp-unit.h (struct comp_unit_head) <read_address>: New
	method.

Change-Id: Ibd6c7235f2e4d5fd88c272cfd2c3d3328618cc56