go/types: expose types.Info.Inferred with -tags=typeparams

Our workaround to get and set types.Info._Inferred makes it harder to
experiment with the new APIs in x/tools.

Instead, just make a copy of the types.Info struct, so that the Inferred
field is accessible when the typeparams build tag is set.

This is a trivially safe change: the only change when not building with
-tags=typeparams is that types.Info._Inferred is removed, and accessing
inferred type information goes through an additional layer of
indirection.

For #46003

Change-Id: I38f2bbb2c80aed28be31d0fe762ccead970476ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/317549


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>