Support: Improve performance of FileOutputBuffer on Windows

We extend an underlying file before mmap'ing it, but it's not needed
on Windows. Extending file is slow on Windows, so we should avoid doing that.
The difference gets larger as the size of an output file gets larger.
It shove off 2 seconds out of 25 seconds when linking chrome.dll with LLD,
for example.

llvm-svn: 231452