Make sure TestNumThreads works with libc++

The problem was that with libc++ the std::unique_lock declaration was
completely inlined, so there was no line table entry in the main.cpp
file to set a breakpoint on. Therefore, the breakpoint got moved to the
next line, but that meant the test would deadlock as the thread would
stop with the lock already held.

I fix that issue by adding a dummy statement before the std::unique_lock
line to anchor the breakpoint.

I think this should fix the issue because of which this test was
disabled on darwin, but someone should verify that before enabling it.

llvm-svn: 335132