This introduces an iterable object which can be used to iterate over
objfiles. It also introduces a generic "next_iterator", which can be
used to iterate over types that have a "next" field.
gdb/ChangeLog
2019-01-09 Tom Tromey <tom@tromey.com>
* common/next-iterator.h: New file.
* objfiles.h (class all_objfiles): New.
(struct objfile_iterator): New.