Add clang thread safety annotations to mutex and lock_guard. Patch by jamesr@google.com.

This adds clang thread safety annotations to std::mutex and
std::lock_guard so code using these types can use these types directly
instead of having to wrap the types to provide annotations. These checks
when enabled by -Wthread-safety provide simple but useful static
checking to detect potential race conditions.
See http://clang.llvm.org/docs/ThreadSafetyAnalysis.html for details.

This patch was reviewed in http://reviews.llvm.org/D14731.

llvm-svn: 263611