[flang][hlfir] Insert casts in hlfir.associate rewrites

When the associated expression came from a moved variable, the
type of the moved variable may not exactly match the hlfir.associate
result and cannot be re-used directly. Insert fir.convert/fir.box_addr
as needed.

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D144557