[sanitizer] Don't call task_for_pid(mach_task_self). NFC.

Calling task_for_pid with mach_task_self is just returning mach_task_self anyway, but it also triggers system warnings (task_for_pid is only supposed to be used by high-privileged executables). NFC.

rdar://problem/39198248

Differential Revision: https://reviews.llvm.org/D51119

llvm-svn: 340587