IR: Make MDNode::dump() useful by adding addresses

It's horrible to inspect `MDNode`s in a debugger.  All of their operands
that are `MDNode`s get dumped as `<badref>`, since we can't assign
metadata slots in the context of a `Metadata::dump()`.  (Why not?  Why
not assign numbers lazily?  Because then each time you called `dump()`,
a given `MDNode` could have a different lazily assigned number.)

Fortunately, the C memory model gives us perfectly good identifiers for
`MDNode`.  Add pointer addresses to the dumps, transforming this:

    (lldb) e N->dump()
    !{i32 662302, i32 26, <badref>, null}

    (lldb) e ((MDNode*)N->getOperand(2))->dump()
    !{i32 4, !"foo"}

into:

    (lldb) e N->dump()
    !{i32 662302, i32 26, <0x100706ee0>, null}

    (lldb) e ((MDNode*)0x100706ee0)->dump()
    !{i32 4, !"foo"}

and this:

    (lldb) e N->dump()
    0x101200248 = !{<badref>, <badref>, <badref>, <badref>, <badref>}

    (lldb) e N->getOperand(0)
    (const llvm::MDOperand) $0 = {
      MD = 0x00000001012004e0
    }
    (lldb) e N->getOperand(1)
    (const llvm::MDOperand) $1 = {
      MD = 0x00000001012004e0
    }
    (lldb) e N->getOperand(2)
    (const llvm::MDOperand) $2 = {
      MD = 0x0000000101200058
    }
    (lldb) e N->getOperand(3)
    (const llvm::MDOperand) $3 = {
      MD = 0x00000001012004e0
    }
    (lldb) e N->getOperand(4)
    (const llvm::MDOperand) $4 = {
      MD = 0x0000000101200058
    }
    (lldb) e ((MDNode*)0x00000001012004e0)->dump()
    !{}

    (lldb) e ((MDNode*)0x0000000101200058)->dump()
    !{null}

into:

    (lldb) e N->dump()
    !{<0x1012004e0>, <0x1012004e0>, <0x101200058>, <0x1012004e0>, <0x101200058>}

    (lldb) e ((MDNode*)0x1012004e0)->dump()
    !{}

    (lldb) e ((MDNode*)0x101200058)->dump()
    !{null}