go/types, types2: improve error messages referencing any

Because any is an a alias, it is naively formatted as interface{} in
error messages. This is a source of verbosity and potential confusion.

We can improve the situation by looking for pointer equality with the
any type. To avoid churn in the importers, do this all at once across
the compiler, go/types, and go/internal/gcimporter. CL 364194 makes the
corresponding change in x/tools/go/internal/gcimporter, allowing the
x/tools trybots to pass.

Fixes #49583

Change-Id: Ib59570937601308483f6273364cc59338f9b8b3b
Reviewed-on: https://go-review.googlesource.com/c/go/+/363974


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>