Move InterlockedBitTestAndSet into its own test file

Summary:
This gives us back the test coverage we lost when I made
MicrosoftExtensions.c x86-only.

Reviewers: majnemer

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4401

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212414 91177308-0d34-0410-b5e6-96231b3b80d8