Migration: clean up handling of type substitutions.

Previously, when the migration tool needed to change one type variable
to another during a substitution, it created a DecortedType to
represent the substitution using
`DecoratedType._forTypeParameterSubstitution`.  This was problematic
because the resulting DecoratedType had a null nullability `node`.
This in turn forced nearly every reference to `DecoratedType.node` to
be followed by a `!` operator.

With this change, we now have a new base class, `SubstitutedType`,
which can either be a `DecoratedType` (representing a fully general
substitution) or a `_TypeVariableReplacement` (representing a simple
change from one type variable to another).  This paves the way for a
follow-up CL that will make `DecoratedType.node` non-nullable.

Change-Id: I7fdaef57994c7678ecd517ab4fe84d00f0d81424
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/261941
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>