go/types, types2: add additional tracing output in type inference
To support analyzing type inference failures.
Change-Id: I47882f6b634d5fcf1834923ec372313323d31a9d
Reviewed-on: https://go-review.googlesource.com/c/go/+/617895
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Robert Griesemer <gri@google.com> Reviewed-by: Tim King <taking@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>