tsan: simplify thread context setting

Currently we set thr->tctx after OnStarted callback
taking thread registry mutex again and searching for the context.
But OnStarted already runs under the thread registry mutex
and has access to the context, so set it in the OnStarted.
This makes code simpler and faster.

Depends on D110132.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D110133