go/types, types2: remove indirection of Named.finite

go/types, types2: remove indirection of Named.finite

This field is now guarded by a state bit. The "unknown" state is hence
indicated by absence of that bit.

Change-Id: I9605538f089dd4fd638e5f0f416cd4b8737b6f88
Reviewed-on: https://go-review.googlesource.com/c/go/+/729041
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>