cmd: add a new goexperiment for redesigned code coverage

cmd: add a new goexperiment for redesigned code coverage

Add a new "coverageredesign" GOEXPERIMENT (currently off by default),
for gating the use of the new code coverage design/implementation.

Updates #51430.

Change-Id: Ia61da869fcd0d61c6163f734e2fe5e3705f37a91
Reviewed-on: https://go-review.googlesource.com/c/go/+/395894
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Austin Clements <austin@google.com>