Revert "IR: MDNode => Value: Update for LLVM API change in r221375"

This reverts commit r221376.

The API change was reverted in r221711.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221712 91177308-0d34-0410-b5e6-96231b3b80d8