go/types: implement type sets with term lists

This is a port of CL 338310 to go/types. It is superficially adjusted
for different error reporting and AST APIs. It also fixes a bug in CL
338310 that only manifests in go/types (TestFixedbugs/issue39755.go2)
due to go/types preserving untyped nil. In that CL, operand.go is
checking if optype is a TypeParam, which can never be the case. A fix
for types2 will be mailed in a separate CL.

Change-Id: Icf3394e74baec536842267d99f7511d25ab32a8a
Reviewed-on: https://go-review.googlesource.com/c/go/+/342331


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>