[flang][hlfir] Cast actual cst len character to stmt func dummy type (#68598)

When calling a statement function with a character actual argument with
a constant length mismatching the dummy length, HLFIR lowering created
an hlfir.declare with the actual argument length for the dummy, causing
bugs when lowering the statement function expression.

Ensure character dummies are always cast to the dummy type when lowering
dummy declarations.