go/types, types2: add tests for literals in type parameter lists

Add tests that verify consistent behavior of go/types and types2 with
respect to potentially ambiguous type parameter lists.

For #49482

Change-Id: I3386d4fa3eb91f2a8ea0987372ca40a6962de886
Reviewed-on: https://go-review.googlesource.com/c/go/+/385756


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>