AsmPrinter: Change DIEValueList to a subclass of DIE, NFC

Rewrite `DIEValueList` as a subclass of `DIE`, renaming its API to match
`DIE`'s.  This is preparation for changing `DIEBlock` and `DIELoc` to
stop inheriting from `DIE` and inherit directly from `DIEValueList`.

I thought about leaving this as a has-a relationship (and changing
`DIELoc` and `DIEBlock` to also have-a `DIEValueList`), but that seemed
to require a fair bit more boilerplate and I think it needed more
changes to the `DwarfUnit` API than this will.

No functionality change intended here.

llvm-svn: 243854