Desugarer: Bring existentials in scope when substituting into record GADTs

This fixes an outright bug in which the desugarer did not bring the existentially quantified type variables of a record GADT into in_subst's in-scope set, leading to #20278 (closed). It also addresses a minor inefficiency in which out_subst was made into a substitution when a simpler TvSubstEnv would suffice.

Fixes #20278 (closed).