Use gdb_bfd_sections in dwarf2/read.c

Use gdb_bfd_sections in dwarf2/read.c

This changes some functions in dwarf2/read.c to avoid
bfd_map_over_sections, in favor of iteration.

gdb/ChangeLog
2020-09-19  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (locate_dwz_sections): Change parameters.
	(dwarf2_get_dwz_file): Use foreach.
	(dwarf2_locate_dwo_sections): Change parameters.
	(open_and_init_dwo_file): Use foreach.
	(dwarf2_locate_common_dwp_sections): Change parameters.
	(open_and_init_dwp_file): Use foreach.