Add note on bad compilers

Author: thurstond

compiler-rt/test/asan/TestCases/zero_alloc.cpp

@@ -1,5 +1,7 @@
 // RUN: %clang_asan -Wno-alloc-size -fsanitize-recover=address %s -o %t && %env_asan_opts=halt_on_error=0 %run %t 2>&1 | FileCheck %s
 
+// ASan doesn't catch this because internally it translates 0-byte allocations
+// into 1-byte
 // XFAIL: *
 
 #include <malloc.h>