Work around binutils/15021.

	Work around binutils/15021.
	* dwarf2read.c (dwarf2_per_cu_data): Split imported_symtabs and
	type_unit_group out of union s.  All uses updated.
	(read_index_from_section): Watch for index version 8.
	(follow_die_sig): If using .gdb_index version <= 7, record the TU as
	an imported symtab.
	(write_psymtabs_to_index): Increment version number to 8.

	doc/
	* gdb.texinfo (Index Section Format): Document .gdb_index version 8.