go/parser: match syntax parser error for unnamed type parameters

go/parser: match syntax parser error for unnamed type parameters

For #54511.

Change-Id: I1ae391b5f157bf688f9f31b1577c90e681b6df26
Reviewed-on: https://go-review.googlesource.com/c/go/+/426655
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Robert Griesemer <gri@google.com>