[COFF] Support -gsplit-dwarf for COFF on Windows

D152340 has split WinCOFFObjectWriter to WinCOFFWriter. This patch adds
another WinCOFFWriter as DwoWriter to write Dwo sections to dwo file.
Driver options are also updated accordingly to support -gsplit-dwarf in
CL mode.

e.g. $ clang-cl  -c -gdwarf -gsplit-dwarf foo.c

Like what -gsplit-dwarf did in ELF, using this option will create DWARF object
(.dwo) file. DWARF debug info is split between COFF object and DWARF object
file. It can reduce the executable file size especially for large project.

Reviewed By: skan, MaskRay

Differential Revision: https://reviews.llvm.org/D152785