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>

src/internal/goexperiment/exp_coverageredesign_off.go

@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build !goexperiment.coverageredesign
+// +build !goexperiment.coverageredesign
+
+package goexperiment
+
+const CoverageRedesign = false
+const CoverageRedesignInt = 0

src/internal/goexperiment/exp_coverageredesign_on.go

@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build goexperiment.coverageredesign
+// +build goexperiment.coverageredesign
+
+package goexperiment
+
+const CoverageRedesign = true
+const CoverageRedesignInt = 1

src/internal/goexperiment/flags.go

@@ -86,4 +86,8 @@ type Flags struct {
 	// has been broken out to its own experiment that is disabled
 	// by default.
 	HeapMinimum512KiB bool
+
+	// CoverageRedesign enables the new compiler-based code coverage
+	// tooling.
+	CoverageRedesign bool
 }