Ensure code complete with !LoadExternal sees all local decls.

Summary:
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.

While here:
 - 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.

Reviewers: ilya-biryukov

Subscribers: cfe-commits, rsmith

Differential Revision: https://reviews.llvm.org/D42077

llvm-svn: 322548