[msan] Break optimization in memccpy tests

After D116148 the memccpy gets optimized away and the expected
uninitialized memory access does not occur.

Make sure the call does not get optimized away.

Author: nikic

compiler-rt/lib/msan/tests/msan_test.cpp

@@ -1560,6 +1560,7 @@ TEST(MemorySanitizer, memccpy_nomatch_positive) {
   char* y = new char[5];
   strcpy(x, "abc");
   EXPECT_UMR(memccpy(y, x, 'd', 5));
+  break_optimization(y);
   delete[] x;
   delete[] y;
 }
@@ -1570,6 +1571,7 @@ TEST(MemorySanitizer, memccpy_match_positive) {
   x[0] = 'a';
   x[2] = 'b';
   EXPECT_UMR(memccpy(y, x, 'b', 5));
+  break_optimization(y);
   delete[] x;
   delete[] y;
 }