[dev.typeparams] cmd/compile/internal/types2: make predeclared "any" alias for interface{}

If we ever decide to permit the use of the predeclared identifier
"any" in lieu of interface{}, it must be an alias for interface{}.

Change-Id: Ic751d7f9b61133fb57625f56ce95d99f034b32c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/285132


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