[flang] Set correct box type when emboxing temporary in array value copy pass

In some cases, the base type is already a fir.box type. This patch updates
the code to set the result box type to fir.box<T>, where T is the type
of the allocated temp.

This was reported in https://github.com/llvm/llvm-project/issues/59342

Reviewed By: jeanPerier, tblah

Differential Revision: https://reviews.llvm.org/D139401