[scudo] Accessing PossibleRegions requires lock

It is preferable to use `std::shared_mutex` style mutex. Will switch to
using it when it's available.

Differential Revision: https://reviews.llvm.org/D144691

GitOrigin-RevId: 6860d1c6e44f3e0a0d182f58b3848f720b5ade59
1 file changed