[lld/COFF] Add /dwodir to enable DWARF fission with LTO

This patch added /dwodir to lld/COFF which is equivalent to lld/ELF
option -plugin-opt=dwo_dir=. This option tells LTO backend to create
dwo directory and files and all dwo files will be in it. Otherwise all
dwarf sections will be embeded into image even if -gsplit-dwarf is
specified when using LTO.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D154070