Move the AddressPool from DwarfFile to DwarfDebug.

There's only ever one address pool, not one per DWARF output file, so
let's just have one.

(similar refactoring of the string pool to come soon)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207026 91177308-0d34-0410-b5e6-96231b3b80d8