[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

GitOrigin-RevId: 636a1cfdcdb8af750a33a6dd2c5d6f84a59aa1c3
1 file changed