[FIR/IR generator] Make AbstractField generic over itself

This will make some code more type-safe.

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

@@ -163,7 +163,7 @@ sealed class Field(
     override val name: String,
     override var isMutable: Boolean,
     val isChild: Boolean,
-) : AbstractField() {
+) : AbstractField<Field>() {
     var baseDefaultValue: String? = null
     var baseGetter: String? = null