[FIR generator] Replace AbstractElement with ElemenOrRef

In the FIR generator, the `AbstractElement` class was used to
represent either an element type without type arguments applied
(using the `Element` subclass), or an element type with applied type
arguments (using the `ElementWithArguments` subclass).

Instead, it is more logical to use the `Element` class to always
represent a non-parameterized element type, and for a parameterized
element type use the `ElementRef` class, just like we do
in the IR generator.

compiler/ir/ir.tree/tree-generator/src/org/jetbrains/kotlin/ir/generator/model/Model.kt

@@ -79,6 +79,8 @@ class Element(
     }
 }
 
+// FIXME: Replace this class with `typealias ElementRef = org.jetbrains.kotlin.generators.tree.ElementRef<Element, Field>`
+// FIXME: as soon as Element implements the org.jetbrains.kotlin.generators.tree.AbstractElement interface.
 data class ElementRef(
     val element: Element,
     override val args: Map<NamedTypeParameterRef, TypeRef> = emptyMap(),