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.