[AIX][XCOFF][Patch1] Provide decoding trace back table information API for...

[AIX][XCOFF][Patch1] Provide decoding trace back table information API for xcoff object file for llvm-objdump -d

SUMMARY:

1. This patch provided API for decoding the traceback table info and unit test for the these API.

2. Another patchs will do the following things:
2.1 added a new option --traceback-table to decode the trace back table information for xcoff object file when
using llvm-objdump to disassemble the xcoff objfile.

2.2 print out the  traceback table information for llvm-objdump.

Reviewers:  Jason liu, Hubert Tong, James Henderson

Differential Revision: https://reviews.llvm.org/D81585