[PATCH] Flush output streams upon process termination

[Zachary Turner]

@thakis: Done.   https://connect.microsoft.com/VisualStudio/feedback/details/888893/stdout-and-stderr-are-not-necessarily-flushed-upon-clean-process-exit
@rnk: Interesting.  I am incllined to say that yes, that is definitely the same bug.  

I think even if we do write to c stdio handlers from other atexit handlers, it should still be fine.  Calls to atexit() handlers are serialized, so worst case scenario some *additional* data that was already not getting flushed, still doesn't get flushed.  It's still better than the current flakiness in all cases.

Also, I've confirmed with at least 2 other windows engineers that they have experienced both stderr and stdout not flushing on process termination, so apparently this is a "known issue".

http://reviews.llvm.org/D4021