Report reverse dependencies for dangling rules.

To help find what pulls them in.

Test: m checkbuild on a tree that exhibited the problem in
  https://android-build.googleplex.com/builds/pending/P13607612/aosp_crosshatch-userdebug/latest/view/logs/build_error.log

Change-Id: I59f3c98f21e5fb45dbd0aa814400f734d769a442