[flang][HLFIR] compute elemental function result length parameters (#93983)

Prepare the argument and map them to their corresponding dummy symbol in
order to lower the specification expression of the function result.

Extract the preparation of arguments according to the interface to its
own function to be reused.

It seems there is no need to conditionally compute the length on the
input since all the information comes from the CharBoxValue or the
descriptor for cases where the number of element could be 0.