[TSan] Build TSan-instrumented version of libcxx and use it in lit tests.

TSan can produce false positives in code that uses C++11 threading,
as it doesn't see synchronization inside standard library. See
http://lists.cs.uiuc.edu/pipermail/cfe-dev/2014-February/035408.html
for an example of such case.

We may build custom TSan-instrumented version libcxx to fight with that.
This change adds build rules for libcxx_tsan and integrates it into
testing infrastructure.