[asan] Clean up some confusing code in

`test/asan/TestCases/Darwin/segv_read_write.c`

* The `fd` arg passed to `mmap()` should be `-1`. It is not defined
what passing `0` does on Darwin.

* The comment about the shadow memory doesn't make any sense to me,
so I'm removing it.

Differential Revision: https://reviews.llvm.org/D44579

llvm-svn: 341307