Migration: store decorated type parameter bounds separately from other element decorated types.

We will need some special treatment for decorated type parameter
bounds, because sometimes we will need to create type parameters
(e.g. when comparing decorated types).  So we store the bounds of
non-ephemeral type parameters (the ones whose enclosing element is
non-null) in the Variables data structure, and we store the bounds of
ephemeral type parameters (the ones whose enclosing element is null)
in an expando.

Change-Id: I918c9baffedab9a8871ecea37c8101b5ef44a5d9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/112743
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>