[PATCH] Test that the single-threaded lit feature is available iff the corresponding guard is #defined

[Eric Fiselier]

I wasn't suggesting making it a `.pass.cpp` test. Just making it fail during compilation instead. I would prefer using a `#error` directive so the output of the test failure is clearer, but it's up to you.

Otherwise, LGTM.

http://reviews.llvm.org/D6006