Convert read_offset to method on comp_unit_head

This changes one of the read_offset overloads to be a method on
comp_unit_head.

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

	* dwarf2/read.c (read_attribute_value, read_indirect_string)
	(read_indirect_line_string): Update.
	* dwarf2/comp-unit.c (read_offset): Remove.
	(read_comp_unit_head): Update.
	* dwarf2/comp-unit.h (struct comp_unit_head) <read_offset>: New
	method.
	(read_offset): Don't declare.

Change-Id: Ia595702a5748337b7c031352bc437956baab9990