Currently, `dyn_cast<XCOFFObjectFile>` always does cast and returns a pointer,
even when we pass `ELF`/`Wasm`/`Mach-O` or `COFF` instead of `XCOFF`.
It happens because `XCOFFObjectFile` class does not implement `classof`.
I've fixed it and added a unit test.
Differential revision: https://reviews.llvm.org/D86542
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
Georgii Rymar