[NFC][sanitizer] Track progress of populating the block

In multi-threaded application concurrent StackStore::Store may
finish in order different from assigned Id. So we can't assume
that after we switch writing the next block the previous is done.

The workaround is to count exact number of uptr stored into the block,
including skipped tail/head which were not able to fit entire trace.

Depends on D114490.

Reviewed By: morehouse

Differential Revision: https://reviews.llvm.org/D114493