[flang] Handle correctly optional intrinsic scalar to unlimited polymorphic optional

When an optional intrinsic scalar is passed to a function expecting an
unlimited polymorphic dummy argument, the presence test must be done
before the emboxing otherwise it will result in a program crash.

Depends on D143888

Reviewed By: jeanPerier, PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D143889