[tsan] First commit of ThreadSanitizer (TSan) run-time library.

Algorithm description: http://code.google.com/p/thread-sanitizer/wiki/ThreadSanitizerAlgorithm

Status:
The tool is known to work on large real-life applications, but still has quite a few rough edges.
Nothing is guaranteed yet.

The tool works on x86_64 Linux.
Support for 64-bit MacOS 10.7+ is planned for late 2012.
Support for 32-bit OSes is doable, but problematic and not yet planed.

Further commits coming:
  - tests
  - makefiles
  - documentation
  - clang driver patch

The code was previously developed at http://code.google.com/p/data-race-test/source/browse/trunk/v2/
by Dmitry Vyukov and Kostya Serebryany with contributions from
Timur Iskhodzhanov, Alexander Potapenko, Alexey Samsonov and Evgeniy Stepanov.

llvm-svn: 156542