[cfe] Add _BuilderFactory

This moves some of the functionality for creating Builders from
Fragments into a _BuilderFactory.

Change-Id: Ib12a1d41a325bce767a2c65438a4751b188671b2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/435782
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>

Author: johnniwinther