[DebugInfo] Make children iterator bidirectional

Make the DIE iterator bidirectional so we can move to the previous
sibling of a DIE.

Differential revision: https://reviews.llvm.org/D49173

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@336823 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed