`_` suffixed types aren't properly escaped in union types

[Paul Daniels]

If the typename has a _ at the end and is then part of a union:

type Subscription_ {
  id: ID!
}

union SubscriptionResponse = Subscription_ | SubscriptionFailure

The resulting code gen client doesn't compile because it creates a selector that looks like this:

(onSubscription_: SelectionBuilder[`Subscription_`, A], onSubscriptionFailure: SelectionBuilder[SubscriptionFailure, A])

The scala2 compiler doesn't like <name>_: constructs so it throws during schema creation.