Remove field_info::nfields member

I noticed that there's no real reason to have field_info::nfields in
the DWARF reader.  It simply mirrors information that is already
available.  This patch removes it, in favor of a convenience method.

gdb/ChangeLog
2020-02-27  Tom Tromey  <tromey@adacore.com>

	* dwarf2/read.c (struct field_info) <nfields>: Now a method, not a
	member.
	(dwarf2_add_field): Don't update nfields.
	(dwarf2_attach_fields_to_type, process_structure_scope): Update.