[PATCH v1] c++: follow DR 2386 and update implementation of get_tuple_size [PR110216]

DR 2386 updated the tuple_size requirements for structured binding and it now requires tuple_size to be considered only if std::tuple_size names a complete class type with member value. GCC before this patch does not follow the updated requrements, and this patch is intended to implement it.

[PATCH] c++: follow DR 2386 and update implementation of get_tuple_size [PR110216]

DR 2386 updated the tuple_size requirements for structured binding and it now requires tuple_size to be considered only if std::tuple_size names a complete class type with member value. GCC before this patch does not follow the updated requrements, and this patch is intended to implement it.