[scudo] Make Scudo compile for C++20

[cryptoad]

Summary

In C++20 compound assignment to volatile (here LocalData[I]++) is
deprecated, so mutex_test.cpp fails to compile.

Simply changing it to LocalData[I] = LocalData[I] + 1 fixes it.

[hctim]

@ayermolo I think your rule (https://reviews.llvm.org/H844#2954) marks you as a blocking reviewer on every patchset.

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

for (scudo::u32 I = 0; I < Size; I++) {
       Data[I]++;
     }
     Mutex.unlock();
   }
 
   void backoff() {
     volatile T LocalData[Size] = {};
     for (scudo::u32 I = 0; I < Size; I++) {
-      LocalData[I]++;
+      LocalData[I] = LocalData[I] + 1;
       EXPECT_EQ(LocalData[I], 1U);
     }
   }
 
 private:
   static const scudo::u32 Size = 64U;
   typedef scudo::u64 T;
   scudo::HybridMutex &Mutex;