[lsan] Expose Frontier object to OS-specific LockStuffAndStopTheWorld callback

This is a small refactoring to prepare for porting LSan to Fuchsia.
On Fuchsia, the system supplies a unified API for suspending threads and
enumerating roots from OS-specific places like thread state and global data
ranges. So its LockStuffAndStopTheWorld implementation will make specific
callbacks for all the OS-specific root collection work before making the
common callback that includes the actual leak-checking logic.

Patch By: mcgrathr

Differential Revision: https://reviews.llvm.org/D72988