This removes the ALL_COMPUNITS, replacing its uses with two nested
ranged for loops.

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

	* symtab.c (lookup_objfile_from_block)
	(find_pc_sect_compunit_symtab, search_symbols)
	(default_collect_symbol_completion_matches_break_on): Use
	objfile_compunits.
	* objfiles.h (ALL_COMPUNITS): Remove.
	* maint.c (count_symtabs_and_blocks): Use objfile_compunits.
	* cp-support.c (add_symbol_overload_list_qualified): Use
	objfile_compunits.
	* ada-lang.c (ada_collect_symbol_completion_matches)
	(ada_add_global_exceptions): Use objfile_compunits.