Convert IS_TYPE_UNIT_GROUP to method

This converts the IS_TYPE_UNIT_GROUP to a method on
dwarf2_per_cu_data.

gdb/ChangeLog
2020-02-24  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.h (struct dwarf2_per_cu_data) <type_unit_group_p>:
	New method.
	* dwarf2/read.c (IS_TYPE_UNIT_GROUP): Remove.
	(dw2_do_instantiate_symtab, dw2_get_file_names)
	(build_type_psymtab_dependencies, load_full_type_unit): Update.