noload_lookups() was too lazy: in addition to avoiding external decls, it
avoided populating the lazy lookup structure for internal decls.
This is the right behavior for the existing callsite in ASTDumper, but I think
it's not a very useful default, so we populate it by default.
- remove an unused test file accidentally added in r322371.
- remove lookups_begin()/lookups_end() in favor of lookups().begin(), which is more common and more efficient.