Factor the Unit section parsing into the DWARFUnitSection class.

Summary: No functional change.

Reviewers: dblaikie, samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5522
[dwarfdump] Print the name for referenced specification of abstract_origin DIEs.

Reviewers: dblaikie, samsonov, echristo, aprantl

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5466