[obj2yaml] Refactor the .debug_pub* sections dumper.

It's good to reuse the DWARF parser in lib/DebugInfo so that we don't
need to maintain a separate parser in client side (obj2yaml). Besides,
A test case is added whose length field is a very huge value which makes
obj2yaml stuck when parsing the section.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D86192