MC: rename WinCOFFStreamer and move declaration out-of-line

This is in preparation for promoting WinCOFFStreamer to a base class which will
be shared by the X86 and ARM specific target COFF streamers.  Also add a new
getOrCreateSymbolData interface (like MCELFStreamer) for the ARM COFF Streamer.
This makes the COFFStreamer more similar to the ELFStreamer.

llvm-svn: 207343