Cpp: Fix the unused parameter warning in the sempred function.

[0xFireWolf]

Consider the following simple grammar:

Expression : Expression '*' Expression
           | Expression '+' Expression

ANTLR will generate the following member function:

bool SampleParser::expressionSempred(ExpressionContext *_localctx, size_t predicateIndex)
{
    switch (predicateIndex) 
    {
        case 0: return precpred(_ctx, 5);
        case 1: return precpred(_ctx, 4);

        default:
        break;
    }
    return true;
}

The first parameter is never used in the function body, resulting in an unused parameter warning. This PR fixes the warning by adding the attribute [[maybe_unused]] which is available as of C++17.

Thank you.

[parrt]

Hi. thanks. can you remind me of the min C++ version we require?

[0xFireWolf]

Hi. thanks. can you remind me of the min C++ version we require?

I think it is C++17.