[lsan] Don't crash on ThreadRegistry::threads_ data race

Comment "No lock needed" in CurrentThreadContext was wrong.
Concurent ThreadRegistry::CreateThread can resize and relocate
ThreadRegistry::threads_ the same time CurrentThreadContext reads it.

To mitigate lock cost we store ThreadContext* instead of tid in
THREADLOCAL cache, we can tid from the ThreadContext*.

Reviewed By: kstoimenov, MaskRay

Differential Revision: https://reviews.llvm.org/D148281