go/types, types2: put Named.finite behind Named.mu

go/types, types2: put Named.finite behind Named.mu

This change adds another leaf state to named types which indicates
whether the type's size finiteness is known.

Without this, writes to Named.finite can result in a clobbered value.
While benign in terms of functionality, it triggers the race detector.

Fixes #76773

Change-Id: I2ac3d8d6f8be55a8120598daecb3e78aa7df5f30
Reviewed-on: https://go-review.googlesource.com/c/go/+/729021
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Mark Freeman <markfreeman@google.com>