tsan: fork runtime

Fork the current version of tsan runtime before commiting
rewrite of the runtime (D112603). The old runtime can be
enabled with TSAN_USE_OLD_RUNTIME option.
This is a temporal measure for emergencies and is required
for Chromium rollout (for context see http://crbug.com/1275581).
The old runtime is supposed to be deleted soon.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D115223