Apollo Client Codegen does not take into account Field Policies (Apollo Client 3.0)

**Intended outcome:**

Types generated by apollo client:codegen should take into account transformations resulting from [Field Policies](https://www.apollographql.com/docs/react/caching/cache-field-behavior/#the-read-function)

**Actual outcome:**

Types generated by apollo client:codegen do not reflect transformations resulting from Field Policies.

**How to reproduce the issue:**

Create a 'read' Field Policy that changes the type of a given cached field. The codegen type will not reflect this.

**Versions**

Tooling: 2.30.1
Apollo Client: 3.0.0

Start at the apollo client:codegen entry point and review how generated types are derived from cached fields. Reproduce the issue with a Field Policy whose read function changes a field type, then verify that the generated type reflects that transformation.

Written by the indexing model from the issue text.