[flang] Embox derived-type directly to fir.class without fir.convert

non-polymorphic derived-type can call type-bound procedure with passed-object.
In that case, the derived-type is emboxed in order to be passed to the call.
Until now the emboxing was done to a fir.box followed by a fir.convert.
This patch update the createBox function so that we can directly embox to
a fir.class and avoid the extra fir.convert.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D135790