gdb/dwarf: unique_ptr cleanup

Throughout gdb/dwarf2, use `*_up` typedefs.  Add a few missing typedefs,
and move some so they are, ideally, just after the corresponding class.

Change-Id: Iab5cd8fc2e9989d4bd8d4868586703c2312f254f
Approved-By: Tom Tromey <tom@tromey.com>