[RemoveDIs][NFC] Add DPLabel class [2/3] (#82376)

Patch 2 of 3 to add llvm.dbg.label support to the RemoveDIs project. The
patch stack adds the DPLabel class, which is the RemoveDIs
llvm.dbg.label
equivalent.

       1. Add DbgRecord base class for DPValue and the not-yet-added
          DPLabel class.
    -> 2. Add the DPLabel class.
       3. Enable dbg.label conversion and add support to passes.

This will be used (and tested) in the final patch(es), coming next.