[scudo] Make Scudo compile for C++20

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.

Differential Revision: https://reviews.llvm.org/D117359

lib/scudo/standalone/tests/mutex_test.cpp

@@ -43,7 +43,7 @@ class TestData {
   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);
     }
   }