[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