[dev.typeparams] go/types: don't expose the TypeSet API for 1.18

The TypeSet API is very new and probably not necessary to expose outside
of go/types, at least for 1.18. Users can check whether a type is
contained within a type set via Implements, and can access the
representation of the type set via the embedded Unions.

Change-Id: Icc7355285785bee5aa7a8fe74052bcb0fedcd0a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/341289


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>