[flang][hlfir] Lower char length inquiry via hlfir.get_length.

ApplyOp provides the type parameters in its argument, so we can
take it from there. For hlfir.expr block arguments (such as with
user-defined assignments) we use hlfir.get_length in lowering.

Depends on D154561

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D154562