[flang] Prevent crash from unfoldable TRANSFER() (llvm#92282)

When the MOLD= argument's type is polymorphic, the type of the result
cannot be known at compilation time, so the call cannot be folded even
when the SOURCE= is constant.

Fixes llvm#92264.

Author: klausler

flang/lib/Evaluate/fold.cpp

@@ -272,6 +272,7 @@ std::optional<Expr<SomeType>> FoldTransfer(
     }
   }
   if (sourceBytes && IsActuallyConstant(*source) && moldType && extents &&
+      !moldType->IsPolymorphic() &&
       (moldLength || moldType->category() != TypeCategory::Character)) {
     std::size_t elements{
         extents->empty() ? 1 : static_cast<std::size_t>((*extents)[0])};