[flang][hlfir] Support polymorphic hlfir.expr values.

This patch sets 'polymorphic' attribute of hlfir::ExprType when
the value is created from a polymorphic entity.
Memoization of such ExprType involves creating a mutable descriptor
on the stack, which is initialized (as a null box) and passed to
AllocatableApplyMold with the mold being the entity from which
the ExprType value is being created.

This patch fixes "creating polymorphic temporary" TODO and also
several cases of "'fir.convert' op invalid type conversion" error.

Reviewed By: tblah

Differential Revision: https://reviews.llvm.org/D155541