[IR] Make IrTypeArgument a sealed interface

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/export/TransitiveExportCollector.kt

@@ -66,7 +66,6 @@ class TransitiveExportCollector(val context: JsIrBackendContext) {
             is IrType -> substitute(typeSubstitutionMap)
             is IrTypeProjection -> type.substitute(typeSubstitutionMap)
             is IrStarProjection -> context.irBuiltIns.anyNType
-            else -> error("Unexpected ir type argument")
         }
     }
 

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/utils/IrTypeUtils.kt

@@ -39,7 +39,6 @@ fun IrType.asString(): String = when (this) {
 private fun IrTypeArgument.asString(): String = when (this) {
     is IrStarProjection -> "*"
     is IrTypeProjection -> variance.label + (if (variance != Variance.INVARIANT) " " else "") + type.asString()
-    else -> error("Unexpected kind of IrTypeArgument: " + javaClass.simpleName)
 }
 
 private fun IrClassifierSymbol.asString() = when (this) {