DebugInfo: partially revert cleanup committed in r209680

I'm not sure exactly where/how we end up with an abstract DbgVariable
with a null DIE, but we do... looking into it & will add a test and/or
fix when I figure it out.

Currently shows up in selfhost or compiler-rt builds.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209683 91177308-0d34-0410-b5e6-96231b3b80d8