tsan: don't start background thread after clone

Start the background thread only after fork, but not after clone.
For fork we did this always and it's known to work (or user code has adopted).
But if we do this for the new clone interceptor some code (sandbox2) fails.
So model we used to do for years and don't start the background thread after clone.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D113744