[flang] Update createTempMutableBox to support polymorphic entities

When creating temporary from a polymorphic entity, its dynamic type
information must be carried over to the temporary.
This patch updates createTempMutableBox to support passing a source_box
from which the information will be carried over.
This is tested on the spread intrinsic and follow-up patches will updates
other temporary creation where needed.

Reviewed By: jeanPerier, PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D141667