go/analysis/passes/shadow: add shadow command

In vet, the shadow check is experimental, meaning not on by default.
The new analysis API has no concept of experimental, but you can
easily supply a different checker. By providing a shadow command, we
make it easy for users that want it to run it:

  $ go install golang.org/x/tools/go/analysis/passes/shadow/cmd/shadow
  $ go vet -vettool $(which shadow) my/project

Change-Id: I25dc7f3c830296121c7217e4615e8ff90e1b7c79
Reviewed-on: https://go-review.googlesource.com/c/148565
Reviewed-by: Michael Matloob <matloob@golang.org>

go/analysis/passes/shadow/cmd/shadow/main.go

@@ -0,0 +1,9 @@
+// The shadow command runs the shadow analyzer.
+package main
+
+import (
+	"golang.org/x/tools/go/analysis/passes/shadow"
+	"golang.org/x/tools/go/analysis/singlechecker"
+)
+
+func main() { singlechecker.Main(shadow.Analyzer) }