AsmPrinter: Stop exposing underlying DIE children list, NFC

Update `DIE` API to hide the implementation of `DIE::Children` so we can
swap it out.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238468 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed