Support for C++17 if init-statement

[anoy.]

The following code is correct in my opinion and the variable is definitely used. Nevertheless, it generates a warning, because probably the init-statements in the if (since C++17) are not considered.

void f()
{
    // warning: Unused variable value 'value' [constStatement]
    if (SomeObj value; someFunc(value))
    {
        std::cout << value.str();
    }
}

[Daniel Marjamäki]

Thanks! I can reproduce, I created https://trac.cppcheck.net/ticket/9776