On macOS 26, malloc_zone_statistics() always returns
a malloc_statistics_t with the max_size_in_use field
set to zero. This was an intentional change, so let's
remove calls to this API.
Instead, use the proc_pid_rusage() API and return the
ri_lifetime_max_phys_footprint field of rusage_info_v4.
Finally, remove the llvm::sys::Process::GetMallocUsage()
call on other platforms altogether. It returns the
current value and not the max value, so its misleading.
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