[flang] Allow conversion from boxed derived type to fir.class

This patch updates the fir.convert operation's verifier to allow
conversion from !fir.box<!fir.type<T>> to !fir.class<!fir.type<T>>.

Other conversion involving fir.class are likely needed but will
be added when lowering needs them.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D135445