go/types, go/constant: handle infinities as unknown values

With this change, constant literals (and results of constant
operations) that internally become infinities are represented
externally (to go/constant) as "unknown" values.

The language has no provisions to deal with infinite constants,
and producing unknown values allows the typechecker to report
errors and avoid invalid operations (such as multiplication of
zero with infinity).

Fixes #20583.

Change-Id: I12f36a17d262ff7957b0d3880241b5a8b2984777
Reviewed-on: https://go-review.googlesource.com/c/go/+/271706


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