Small fixes for detect_invalid_pointer_pairs.

[marxin]

Summary

One test-case uses a wrong operation (should be subtraction).
Second test-case should declare a global variables before a tested one
in order to guarantee we will find a red-zone.

[marxin]

Thanks for the review, can you please install the patch submission?

test/asan/TestCases/invalid-pointer-pairs-compare-errors.cc

 // RUN: %clangxx_asan -O0 %s -o %t -mllvm -asan-detect-invalid-pointer-pair
 
 // RUN: %env_asan_opts=detect_invalid_pointer_pairs=1:halt_on_error=0 %run %t 2>&1 | FileCheck %s
 
 #include <assert.h>
 #include <stdlib.h>
 
 int foo(char *p, char *q) {
   return p > q;
 }
 
 char global1[100] = {}, global2[100] = {};
+char __attribute__((used)) smallest_global[5] = {};
 char small_global[7] = {};
+char __attribute__((used)) little_global[10] = {};
+char __attribute__((used)) medium_global[4000] = {};
 char large_global[5000] = {};
+char __attribute__((used)) largest_global[6000] = {};
 
 int main() {
   // Heap allocated memory.
   char *heap1 = (char *)malloc(42);
   char *heap2 = (char *)malloc(42);
 
   // CHECK: ERROR: AddressSanitizer: invalid-pointer-pair
   // CHECK: #{{[0-9]+ .*}} in main {{.*}}invalid-pointer-pairs-compare-errors.cc:[[@LINE+1]]

test/asan/TestCases/invalid-pointer-pairs-subtract-success.cc

 // RUN: %clangxx_asan -O0 %s -o %t -mllvm -asan-detect-invalid-pointer-pair
 
 // RUN: %env_asan_opts=detect_invalid_pointer_pairs=1 %run %t
 
 #include <assert.h>
 #include <stdlib.h>
 
 int bar(char *p, char *q) {
-  return p <= q;
+  return p - q;
 }
 
 char global[10000] = {};
 
 int main() {
   // Heap allocated memory.
   char *p = (char *)malloc(42);
   int r = bar(p, p + 20);