[Driver][MSVC] Support DWARF fission when using LTO on Windows

D154070 has added /dwodir to lld/COFF to tells LTO backend to create dwo
directory and files. This patch makes clang to emit /dwodir to lld when
user specify -gsplit-dwarf with LTO. This behavior is simiar to DWARF
fission with LTO for ELF.

A simple use case:
$clang-cl -c -flto -gdwarf main.c -o main.o
$clang-cl -c -flto -gdwarf a.c -o a.o
$clang-cl -flto -fuse-ld=lld -gdwarf -gsplit-dwarf main.o a.o

This'll generate a dwo file: main.exe_dwo/0.dwo

Reviewed By: mstorsjo, MaskRay, hans

Differential Revision: https://reviews.llvm.org/D154295