[XCOFF] XCOFF constants, MCObjectFileInfo placeholder code for DWARF; NFC

Summary:
This patch introduces the constants defined to identify DWARF sections
in XCOFF into `llvm/BinaryFormat/XCOFF.h` and adds (NFC) placeholder
code to `llvm/lib/MC/MCObjectFileInfo.cpp` where the DWARF sections for
XCOFF are to be set up.

Reviewers: jasonliu, sfertile, daltenty, DiggerLin, Xiangling_L

Reviewed By: jasonliu, sfertile, DiggerLin

Differential Revision: https://reviews.llvm.org/D79220