[scudo] Make Ptr volatile so that the malloc and free calls are not optimized out (llvm#149944)

This fixes the test failure seen in the discussion about
llvm#148066.

(cherry picked from commit fcdcc4e)

Author: fabio-d

compiler-rt/lib/scudo/standalone/tests/wrappers_c_test.cpp

@@ -183,7 +183,8 @@ TEST_F(ScudoWrappersCDeathTest, Malloc) {
   // process doing free(P) is not a double free.
   EXPECT_DEATH(
       {
-        void *Ptr = malloc(Size);
+        // Note: volatile here prevents the calls from being optimized out.
+        void *volatile Ptr = malloc(Size);
         free(Ptr);
         free(Ptr);
       },