Avoid some pointer chasing in DWARF reader

Avoid some pointer chasing in DWARF reader

I noticed a spot in the DWARF reader using "per_objfile->per_bfd",
where a local per_bfd variable had already been created.  Looking
through the file, I found a number of such spots.  This patch changes
them to use the already-existing local, avoiding a bit of excess
pointer chasing.

gdb/ChangeLog
2021-03-26  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (dwarf2_read_debug_names)
	(dwarf2_build_psymtabs_hard, create_addrmap_from_aranges)
	(dw2_debug_names_iterator::next, create_type_unit_group):
	Simplify.