go/types: support type parameters in NewMethodSet

Add handling for TypeParams in NewMethodSet, to bring it in sync with
lookupFieldOrMethod. Also add a test, since we had none. I wanted this
fix to get gopls completion working with type params, but due to the
subtlety of lookupFieldOrMethod, I left a TODO to confirm that there are
no behavioral differences between the APIs.

Updates #45639

Change-Id: I16723e16d4d944ca4ecb4d87fc196815abb6fcff
Reviewed-on: https://go-review.googlesource.com/c/go/+/311455


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