IR: DwarfNode => DebugNode, NFC

IR: DwarfNode => DebugNode, NFC

These things are potentially used for non-DWARF data (see the discussion
in PR22235), so take the `Dwarf` out of the name.  Since the new name
gives fewer clues, update the doxygen to properly describe what they
are.

llvm-svn: 226874