AsmPrinter: Reorganize DIE.h, NFC

An upcoming commit will switch to storing `DIEValue`s by value in a
`DIE`, so move the `DIE` down below all the subclasses it can.  Two
subclasses inherit from `DIE`, so drop those down below `DIE`.

`DIEValue` will become a discriminated union, which contains an instance
of one of the classes that are currently subclasses.  "Big" ones, such
as those that inherit from `DIE`, will continue to be stored by pointer.
This commit does as much of the code motion ahead of time as possible.

llvm-svn: 238346