Use `const` to define objects with values that do not change after construction

Reason

Prevent surprises from unexpectedly changed object values.

Example

void f()
{
    int x = 7;
    const int y = 9;

    for (;;) {
        // ...
    }
    // ...
}

As x is not const, we must assume that it is modified somewhere in the loop.

Enforcement

• Flag unmodified non-const variables.