Ensure internal function is safe

The internal function was unsound, it could cause UB in rare cases where
the user inadvertly stored the returned object in a location that could
outlive the TyCtxt.

In order to make it safe, we now take a type context as an argument to
the internal fn, and we ensure that interned items are lifted using the
provided context.

Thus, this change ensures that the compiler can properly enforce
that the object does not outlive the type context it was lifted to.

Author: celinval