Second half of `bindInvocationExpressionContinued` local function appears unreachable

[jcouv]

The local function is in HasCollectionExpressionApplicableAddMethod.
It is meant to mirror what BindInvocationExpressionContinued is doing in terms of reporting diagnostics and detecting a failure. But it is only used once in context of HasCollectionExpressionApplicableAddMethod and in that context the cases it has to handle are restricted.
We always have !result.Succeeded

[AlekseyTs]

It would be fine to add an assert and simplify the body of the local function based on that. However, it is intentional to have that local function regardless of how call sites we have. This makes it easier to keep the logic in sync.

[jcouv]

This was resolved already:

                // Although this function is modelled after `BindInvocationExpressionContinued`,
                // since `HasCollectionExpressionApplicableAddMethod` uses a placeholder element of type `dynamic`,
                // only the first listed error case can be hit.
                throw ExceptionUtilities.Unreachable();
            }