tsan: add another use-after-free race test

[dvyukov]

Summary

Add a test where a race with free is called during the free itself
(we only have tests where a race with free is caught during the other memory acces).

compiler-rt/test/tsan/free_race3.c

+// RUN: %clang_tsan -O1 %s -o %t && %deflake %run %t | FileCheck %s
+
+#include "test.h"
+
+int *mem;
+
+void *Thread(void *x) {
+  mem[0] = 42;
+  barrier_wait(&barrier);
+  return NULL;
+}
+
+int main() {
+  barrier_init(&barrier, 2);
+  mem = (int*)malloc(100);
+  pthread_t t;
+  pthread_create(&t, 0, Thread, 0);
+  barrier_wait(&barrier);
+  free(mem);
+  pthread_join(t, NULL);
+  return 0;
+}
+
+// CHECK: WARNING: ThreadSanitizer: data race
+// CHECK:   Write of size 8 at {{.*}} by main thread{{.*}}:
+// CHECK:     #0 free
+// CHECK:     #1 main
+// CHECK:   Previous write of size 4 at {{.*}} by thread T1{{.*}}:
+// CHECK:     #0 Thread