[DWARFLinker][NFC] Remove usages of "const object::ObjectFile" from DWARFLinker.

Summary:
DWARFContext has all the required information to access source debug info.
It is not necessary to use "const object::ObjectFile" to create DWARFContext.
Thus this patch removes all usages of "const object::ObjectFile"
from DWARFLinker. Instead, already created DWARFContext is passed
to DWARFLinker. The purpose is to not depend on "const object::ObjectFile".

The patch looks big, but most of changes are renamings and movements.

Testing: it passes "check-all" lit testing. MD5 checksum for clang .dSYM bundle
matches for the dsymutil with/without that patch.

Reviewers: JDevlieghere, friss, dblaikie, aprantl

Reviewed By: JDevlieghere

Subscribers: hiraditya, llvm-commits

Tags: #llvm, #debug-info

Differential Revision: https://reviews.llvm.org/D75029