go/types, types2: disallow multiple blank type parameters

Work-around for #50481: report an error for multiple
blank type parameters. It's always possible to use
non-blank names in those cases.

We expect to lift this restriction for 1.19.

For #50481.

Change-Id: Ifdd2d91340aac1da3387f7d80d46e44f5997c2a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/376058


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Dan Scales <danscales@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>