Bitcode: Add `OLD_` prefix to metadata node records

I'm about to change these, so move the old ones out of the way.

Part of PR21532.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224070 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed