[dwarfdump] Do not apply relocations in mach-o files if there is no LoadedObjectInfo.

Not only do we not need to do anything to read correct values from the
object files, but the current logic actually wrongly applies twice the
section base address when there is no LoadedObjectInfo passed to the
DWARFContext creation (as the added test shows).

Simply do not apply any relocations on the mach-o debug info if there is
no load offset to apply.

llvm-svn: 245807