reflect: add Value.{Comparable,Equal}

reflect: add Value.{Comparable,Equal}

For #46746

Change-Id: I879124974cdb55932cd9d07d3b384d49d5059857
Reviewed-on: https://go-review.googlesource.com/c/go/+/423794
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>