Add ThreadSanitizer debugging support.

This patch adds ThreadSanitizer support into LLDB:
- Adding a new InstrumentationRuntime plugin, ThreadSanitizerRuntime, in the same way ASan is implemented.
- A breakpoint stops in `__tsan_on_report`, then we extract all sorts of information by evaluating an expression. We then populate this into StopReasonExtendedInfo.
- SBThread gets a new API, SBThread::GetStopReasonExtendedBacktraces(), which returns TSan’s backtraces in the form of regular SBThreads. Non-TSan stop reasons return an empty collection.
- Added some test cases.

Reviewed by Greg Clayton.

llvm-svn: 264162