[analyzer] fix #36158, implicit cast missing on covariant return type

Due to API changes, the implicit cast visitor (CodeChecker) was not
finding the correct substituted member type, so the implicit return
type cast was not inserted.

Also fixes another issue that paulberry@ discovered: covariant return
type casts were overwriting the implicit casts from the context type.
The context type should always take priority; implicit covariance
checks are only needed when there isn't a type cast from the context.

Change-Id: I998e8f87371c15175c57864b7e334449268fe339
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/97143
Reviewed-by: Paul Berry <paulberry@google.com>