Introduce a separate debug objfile iterator

Introduce a separate debug objfile iterator

This introduces a new iterator and range adapter for iteration over
the separate debug files of a given objfile.  As in the current
approach, the requested objfile is returned first, followed by the
separate debug objfiles.

gdb/ChangeLog
2019-04-10  Tom Tromey  <tom@tromey.com>

	* symtab.c (lookup_global_symbol_from_objfile)
	(lookup_symbol_in_objfile_from_linkage_name): Use the iterator.
	* objfiles.h (class separate_debug_iterator): New.
	(class separate_debug_range): New.
	(struct objfile) <separate_debug_objfiles>: New method.
	(objfile_separate_debug_iterate): Don't declare.
	* objfiles.c (separate_debug_iterator::operator++): Rename from
	objfile_separate_debug_iterate.
	(objfile_relocate, objfile_rebase, objfile_has_symbols): Use the
	iterator.
	* minsyms.c (lookup_minimal_symbol_by_pc_section): Use the
	iterator.