[DWARFLinkerParallel][Reland] Add interface files, create a skeleton implementation.

This patch creates skeleton implementation for the DWARFLinkerParallel.
It also integrates DWARFLinkerParallel into dsymutil and llvm-dwarfutil,
so that empty DWARFLinker::link() can be called. To do this new command
line option is added "--linker apple/llvm". Additionally it changes
existing DWARFLinker interfaces/implementations to be compatible:
use Error for error reporting for the DWARFStreamer, make DWARFFile to
owner of referenced resources, other small refactorings.

Differential Revision: https://reviews.llvm.org/D147952