tsan_shadow_test: add const qualifier

[thurston]

Summary

Fix build error in https://lab.llvm.org/buildbot/#/builders/57/builds/23839/steps/7/logs/stdio
that I had introduced in https://reviews.llvm.org/D141445

compiler-rt/lib/tsan/tests/unit/tsan_shadow_test.cpp

 }
 
 template <typename Mapping>
 bool broken(uptr what, typename Has<Mapping::kBroken>::Result = false) {
   return Mapping::kBroken & what;
 }
 
 static int CompareRegion(const void *region_a, const void *region_b) {
-  uptr start_a = ((struct Region *)region_a)->start;
-  uptr start_b = ((struct Region *)region_b)->start;
+  uptr start_a = ((const struct Region *)region_a)->start;
+  uptr start_b = ((const struct Region *)region_b)->start;
 
   if (start_a < start_b) {
     return -1;
   } else if (start_a > start_b) {
     return 1;
   } else {
     return 0;
   }