[dev.link] cmd/link: restore -dumpdep in new deadcode pass

Change-Id: I2e52206a95c9463df5661664726a8b3bbf3ad1fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/204825
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/link/internal/ld/deadcode2.go

@@ -18,10 +18,6 @@ import (
 
 var _ = fmt.Print
 
-// TODO:
-// - Debug output:
-//   Emit messages about which symbols are kept or deleted.
-
 type workQueue []loader.Sym
 
 // Implement container/heap.Interface.
@@ -205,6 +201,16 @@ func (d *deadcodePass2) mark(symIdx, parent loader.Sym) {
 		if d.ctxt.Reachparent != nil {
 			d.ldr.Reachparent[symIdx] = parent
 		}
+		if *flagDumpDep {
+			to := d.ldr.SymName(symIdx)
+			if to != "" {
+				from := "_"
+				if parent != 0 {
+					from = d.ldr.SymName(parent)
+				}
+				fmt.Printf("%s -> %s\n", from, to)
+			}
+		}
 	}
 }