AsmPrinter: Remove the vtable-entry from DIEValue

Remove all virtual functions from `DIEValue`, dropping the vtable
pointer from its layout.  Instead, create "impl" functions on the
subclasses, and use the `DIEValue::Type` to implement the dynamic
dispatch.

This is necessary -- obviously not sufficient -- for passing `DIEValue`s
around by value.  However, this change stands on its own: we make tons
of these.  I measured a drop in memory usage from 888 MB down to 860 MB,
or around 3.2%.

(I'm looking at `llc` memory usage on `verify-uselistorder.lto.opt.bc`;
see r236629 for details.)

llvm-svn: 238084