[NFC][COFF] Split WinCOFFObjectWriter to WinCOFFWriter

We'd like to support -gsplit-dwarf for Windows COFF. It requires to
write Dwo and NonDwo sections to different output streams.The original
implementation is not designed to do that and there can be only one
MCObjectWriter. This patch split WinCOFFObjectWriter to WinCOFFWriter so
that:
  1. WinCOFFObjectWriter can create multiple WinCOFFWriter.
  2. Each WinCOFFWriter can separately collect sections it is interested.
  3. Each WinCOFFWriter can write to it's own output stream.

Reviewed By: skan

Differential Revision: https://reviews.llvm.org/D152340