Create dwarf2/comp-unit.[ch]

Create dwarf2/comp-unit.[ch]

This creates the new files dwarf2/comp-unit.[ch], moving
comp_unit_head and helpers to those files.  A couple of functions are
turned into methods, because it was convenient to do so now.

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

	* Makefile.in (COMMON_SFILES): Add dwarf2/comp-unit.c.
	* dwarf2/read.c (struct comp_unit_head): Move to
	dwarf2/comp-unit.h.
	(enum class rcuh_kind): Move to comp-unit.h.
	(get_cu_length, offset_in_cu_p): Now methods on comp_unit_head.
	(read_comp_unit_head, error_check_comp_unit_head)
	(read_and_check_comp_unit_head): Move to comp-unit.c.
	(read_offset, dwarf_unit_type_name): Likewise.
	(create_debug_type_hash_table, read_cutu_die_from_dwo)
	(cutu_reader::cutu_reader, read_call_site_scope)
	(find_partial_die, follow_die_offset): Update.
	* dwarf2/comp-unit.h: New file, from dwarf2read.c.

Change-Id: Id961b9674c0081ed061083c8152c38b27b27388a