Change dwarf2_cu marking to use methods

Change dwarf2_cu marking to use methods

This changes the dwarf2_cu marking functions to be methods on
dwarf2_cu.

gdb/ChangeLog
2021-05-17  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (maybe_queue_comp_unit)
	(dwarf2_per_objfile::age_comp_units): Update.
	(dwarf2_add_dependence, dwarf2_mark_helper, dwarf2_mark): Move to
	dwarf2_cu methods.
	* dwarf2/cu.h (struct dwarf2_cu) <mark, clear_mark, is_marked,
	add_dependence>: New methods.
	<m_dependencies>: Add "m_" prefix.  Now private.
	<m_mark>: Add "m_" prefix.
	* dwarf2/cu.c (dwarf2_cu::dwarf2_cu): Update.
	(dwarf2_mark_helper): New function.
	(dwarf2_cu::mark, dwarf2_cu::add_dependence): New methods.