[flang] Use object before converts in fir.dispatch (#68589)

In case of small interface mismatches between a function on the caller
and callee side, lowering insert converts. These are very often no-ops
at runtime (casting a descriptor to a descriptor), but they matter in
the strongly type IR.

The IR type of an object argument of a fir.dispatch must be the one of
the object, not the one of the callee side dummy, which may differ in
case of mismatches. Otherwise, the codgeneration of fir.dispatch cannot
succeed (it will not access the right binding tables).