[compiler-rt] Fix a warning

This patch fixes:

  compiler-rt/lib/asan/tests/asan_test.cpp:398:27: error: allocation
  of insufficient size '0' for type 'int' with size '4'
  [-Werror,-Walloc-size]

Author: kazutakahirata

compiler-rt/lib/asan/tests/asan_test.cpp

@@ -395,7 +395,10 @@ TEST(AddressSanitizer, ReallocTest) {
   }
   free(ptr);
   // Realloc pointer returned by malloc(0).
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Walloc-size"
   int *ptr2 = Ident((int*)malloc(0));
+#pragma clang diagnostic pop
   ptr2 = Ident((int*)realloc(ptr2, sizeof(*ptr2)));
   *ptr2 = 42;
   EXPECT_EQ(42, *ptr2);