[flang][hlfir] support optional args for intrinsic ops

This also adds support for allocatable non-optional arguments.

Of the transformational intrinsics which currently have their own hlfir
operations, all of the dynamically optional arguments are lowered as
boxes, so that is all that is implemented for now.

One alternative approach would have been to deal with the dynamically
optional arguments when lowering to FIR runtime calls. I decided not to
do this so that any passes working on the intrinsic operations would not
have to know about and handle the dynamically optional arguments.

Depends on D154235

Differential Revision: https://reviews.llvm.org/D154236