tsan: make mem profile data more consistent

We currently query number of threads before reading /proc/self/smaps.
But reading /proc/self/smaps can take lots of time for huge processes
and it's retries several times with different buffer sizes.
Overall it can take tens of seconds. This can make number of threads
significantly inconsistent with the rest of the stats.
So query it after reading /proc/self/smaps.

Depends on D110149.

Reviewed By: melver, vitalybuka

Differential Revision: https://reviews.llvm.org/D110150