[dev.typeparams] cmd/compile/internal/types2: use scope numbers to identify local types

Rather than using a local types' position information, use the type
name's scope numbers to uniquely identify the type from others with
the same name.

We use scope numbers rather than indices (with number-1 == index)
to preserve the invariant that the zero value for a scope is a ready
to use empty scope.

Using scope numbers turned out to be fairly simple after all and
provides a reasonably stable identification which will make debugging
simpler. A scope number series may be a bit longer than a unique ID for
each type name but local types should be reasonably rare.

Also did a minor cleanup in universe.go to ensure Named.orig is set up
correctly (there's still an open TODO but with a work-around).

Change-Id: I73935fa9bd960809fd5c95fe8b8a535c313cfc8f
Reviewed-on: https://go-review.googlesource.com/c/go/+/333192


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