Split type_unit_group

Split type_unit_group

type_unit_group has links to the compunit_symtab and other symtabs.
However, once this object is shared across objfiles, this will no
longer be ok.

This patch introduces a new type_unit_group_unshareable and arranges to
store a map from type unit groups to type_unit_group_unshareable objects
in dwarf2_per_objfile.

YYYY-MM-DD  Tom Tromey  <tom@tromey.com>
YYYY-MM-DD  Simon Marchi  <simon.marchi@efficios.com>

	* dwarf2/read.h (struct type_unit_group_unshareable): New.
	(struct dwarf2_per_objfile) <type_units>: New member.
	<get_type_unit_group_unshareable>: New method.
	* dwarf2/read.c (struct type_unit_group) <compunit_symtab,
	num_symtabs, symtabs>: Remove; move to
	type_unit_group_unshareable.
	(dwarf2_per_objfile::get_type_unit_group_unshareable): New.
	(process_full_type_unit, dwarf2_cu::setup_type_unit_groups)
	(dwarf2_cu::setup_type_unit_groups): Use type_unit_group_unshareable.