COFF: Teach Chunk to write to a mmap'ed output file.

Previously Writer directly handles writes to a file.
Chunks needed to give Writer a continuous chunk of memory.
That was inefficent if you construct data in chunks because
it would require two memory copies (one to construct a chunk
and the other is to write that to a file).

This patch teaches chunk to write directly to a file.
From readability point of view, this is also good because
you no longer have to call hasData() before calling getData().

llvm-svn: 238464