tsan: change ReportMutex::id type to int

We used to use u64 as mutex id because it was some
tricky identifier built from address and reuse count.
Now it's just the mutex index in the report (0, 1, 2...),
so use int to represent it.

Depends on D112603.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D113980