Report missing diagnostic for typeless expression in binary operation

[jcouv]

Fixes #69579

For context, see the other location that uses IsTypelessExpressionAllowedInBinaryOperator (in BinaryOperatorOverloadResolution) and the comments in IsTypelessExpressionAllowedInBinaryOperator.
I'm not exactly sure why we chose this behavior and it seems odd, but I chose to keep the existing behavior (described in comment) and just fix our error reporting.

[jjonescz]

Looks like IsTypelessExpressionAllowedInBinaryOperator returns false if the expression is a kind of typeless expression which is disallowed in binary operator and true if it's allowed or not typeless at all, right? The name confused me as it sounds like it would return true only if the expression is typeless. Should it perhaps be renamed to

• IsDisallowedTypelessExpressionInBinaryOperator and its semantics negated, or
• IsExpressionAllowedInBinaryOperator (with semantics unchanged)?

Also, what happened when this check wasn't here? The GenerateImplicitConversionError failed? #Resolved

[jcouv]

I think the current name is appropriate, looking at its usage in BinaryOperatorOverloadResolution:

        private BinaryOperatorAnalysisResult BinaryOperatorOverloadResolution(...)
        {
            if (!IsTypelessExpressionAllowedInBinaryOperator(kind, left, right))
            {
                resultKind = LookupResultKind.OverloadResolutionFailure;
                originalUserDefinedOperators = default;
                return default(BinaryOperatorAnalysisResult);
            }
            ...

Also, what happened when this check wasn't here?

In DEBUG, the program would assert below (Debug.Assert(!conversion.IsImplicit);).
In RETAIL, the program would fail to report an error and would crash during emit phase. I didn't investigate in details, but I think it's just that GenerateImplicitConversionError doesn't handle the case and so didn't report an error.

[jjonescz]

looking at its usage in BinaryOperatorOverloadResolution

Yeah, I guess there it makes more sense since it's negated.