Also fix a bug in `Context::GetClassType` that previously tried to
complete the class type before returning it. That's not correct --
`GetCompleteTypeImpl` is only appropriate for cases where the type can
trivially be completed and completing it can't fail -- and led to
infinite recursion with this change because we would call `GetClassType`
when producing a diagnostic if completing that class type failed.
This is essentially the result of looking at `.begin()` uses. We also
frequently do `std::shuffle`, but unfortunately STLExtras doesn't
provide a wrapper for that.
Instead, eagerly import only the impls from the api file corresponding
to the current file, if any, because we need those for impl
redeclaration lookup. For all other cases, load only the impls in
libraries that are referenced as part of an impl lookup query.