This removes the ALL_OBJFILES and ALL_FILETABS macros, replacing them
with ranged for loops.

gdb/ChangeLog
2019-01-09  Tom Tromey  <tom@tromey.com>

	* symtab.c (find_line_symtab, info_sources_command)
	(make_source_files_completion_list): Use objfile_compunits.
	* source.c (select_source_symtab): Use objfile_compunits.
	* objfiles.h (struct objfile): Update comment.
	(ALL_OBJFILES): Remove.
	(ALL_FILETABS): Remove.
	* mi/mi-cmd-file.c (mi_cmd_file_list_exec_source_files): Use
	objfile_compunits.