AsmPrinter: Stop inheriting from DIE

Change `DIELoc` and `DIEBlock` to stop inheriting from `DIE`, instead
inheriting from `DIEValueList` to share the value storage API.  This
awkward bit of code-sharing was also fairly confusing: neither `DIELoc`
nor `DIEBlock` represents a `DIE`, so why would they inherit from it?

Aside from the API cleanup, this should improve debug info memory usage
in the backend, since it shaves five pointers off of every `DIELoc` and
`DIEBlock`.  I haven't bothered to measure the savings, though.

llvm-svn: 243858