[flang][hlfir] intrinsic dynamically optional arguments

This adds support for dynamically optional arguments for intrinsics
which do not have their own hlfir operations.

The functions for processing these arguments are mostly the same as the
equivalent functions in ConvertExpr.cpp. I chose not to share
implementations so that HLFIR helpers can be used here. Presumably
ConvertExpr.cpp will go away one day.

Depends on D154236

Differential Revision: https://reviews.llvm.org/D154237