Flush C stdio streams upon process termination

Due to what can only be described as a CRT bug, stdout and amazingly
even stderr are not always flushed upon process termination, especially
when the system is under high threading pressure.  I have found two
repros for this:

1) In lib\Support\Threading.cpp, change sys::Mutex to an
std::recursive_mutex and run check-clang.  Usually between 30 and 40
tests will fail.

2) Add OutputDebugStrings in code that runs during static initialization
and static shutdown.  This will sometimes generate similar failures.

After a substantial amount of troubleshooting and debugging, I found
that I could reproduce this from the command line without running
check-clang.  Simply make the mutex change described in #1, then
manually run the following command many times by running it once, then
pressing Up -> Enter very quickly:

D:\src\llvm\build\vs2013\Debug\bin\c-index-test.EXE -cursor-at=D:\src\llvm\tools\clang\test\Index\targeted-preamble.h:2:15 D:\src\llvm\tools\clang\test\Index\targeted-cursor.c -include D:\src\llvm\build\vs2013\tools\clang\test\Index\Output\targeted-cursor.c.tmp.h -Xclang -error-on-deserialized-decl=NestedVar1      -Xclang -error-on-deserialized-decl=TopVar    | D:\src\llvm\build\vs2013\Debug\bin\FileCheck.EXE D:\src\llvm\tools\clang\test\Index\targeted-cursor.c -check-prefix=PREAMBLE-CURSOR1

Sporadically they will fail, and attaching a debugger to a failed
instance indicates that stdin of FileCheck.exe is empty.

Note that due to the repro in #2, we can rule out a bug in the STL's
mutex implementation, and instead conclude that this is a real flake in
the windows test harness.

Test Plan:
Without patch: Ran check-clang 10 times and saw over 30 Unexpected failures on every run.
With patch: Ran check-clang 10 times and saw 0 unexpected failures across all runs.

Reviewers: rnk

Differential Revision: http://reviews.llvm.org/D4021

Patch by Zachary Turner!