Use ALL_PRIMARY_SYMTABS instead of ALL_SYMTABS in some places.

Use ALL_PRIMARY_SYMTABS instead of ALL_SYMTABS in some places.

Non-primary symtabs share the block vector with their primary symtabs.
In these cases there's no need to use ALL_SYMTABS.

gdb/ChangeLog:

	* ada-lang.c (ada_make_symbol_completion_list): Use
	ALL_PRIMARY_SYMTABS instead of ALL_SYMTABS.
	* symtab.c (lookup_objfile_from_block): Ditto.