[FIR generator] Use TypeRef for denoting types

(instead of `Type` and `Importable`)

This is a step towards commonizing the code generator between
FIR and IR: KT-61970

generators/tree-generator-common/src/org/jetbrains/kotlin/generators/tree/AbstractElement.kt

@@ -8,7 +8,8 @@ package org.jetbrains.kotlin.generators.tree
 /**
  * A common interface representing a FIR or IR tree element.
  */
-interface AbstractElement<Element : AbstractElement<Element, Field>, Field : AbstractField> : FieldContainer, ImplementationKindOwner {
+interface AbstractElement<Element : AbstractElement<Element, Field>, Field : AbstractField> : FieldContainer, ImplementationKindOwner,
+    TypeRef /* TODO: Replace with ElementOrRef */ {
 
     val name: String
 
@@ -18,9 +19,9 @@ interface AbstractElement<Element : AbstractElement<Element, Field>, Field : Abs
 
     val typeArguments: List<TypeArgument>
 
-    val parentsArguments: Map<Element, Map<Importable, Importable>>
+    val parentsArguments: Map<Element, Map<TypeRef, TypeRef>>
 
-    val overridenFields: Map<Field, Map<Importable, Boolean>>
+    val overridenFields: Map<Field, Map<Field, Boolean>>
 
     val isSealed: Boolean
         get() = false

generators/tree-generator-common/src/org/jetbrains/kotlin/generators/tree/AbstractField.kt

@@ -9,7 +9,10 @@ abstract class AbstractField : Importable {
 
     abstract val name: String
 
-    open val arguments = mutableListOf<Importable>()
+    val typeRef: TypeRef
+        get() = type(packageName!!, type).withArgs(*arguments.toTypedArray()).copy(nullable = nullable)
+
+    open val arguments = mutableListOf<TypeRef>()
 
     abstract val nullable: Boolean
 

generators/tree-generator-common/src/org/jetbrains/kotlin/generators/tree/StandardTypes.kt

@@ -0,0 +1,10 @@
+/*
+ * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.generators.tree
+
+object StandardTypes {
+    val map = type<Map<*, *>>()
+}

generators/tree-generator-common/src/org/jetbrains/kotlin/generators/tree/TypeArgument.kt

@@ -6,11 +6,11 @@
 package org.jetbrains.kotlin.generators.tree
 
 sealed class TypeArgument(val name: String) {
-    abstract val upperBounds: List<Importable>
+    abstract val upperBounds: List<TypeRef>
 }
 
-class SimpleTypeArgument(name: String, val upperBound: Importable?) : TypeArgument(name) {
-    override val upperBounds: List<Importable> = listOfNotNull(upperBound)
+class SimpleTypeArgument(name: String, val upperBound: TypeRef?) : TypeArgument(name) {
+    override val upperBounds: List<TypeRef> = listOfNotNull(upperBound)
 
     override fun toString(): String {
         var result = name
@@ -21,7 +21,7 @@ class SimpleTypeArgument(name: String, val upperBound: Importable?) : TypeArgume
     }
 }
 
-class TypeArgumentWithMultipleUpperBounds(name: String, override val upperBounds: List<Importable>) : TypeArgument(name) {
+class TypeArgumentWithMultipleUpperBounds(name: String, override val upperBounds: List<TypeRef>) : TypeArgument(name) {
     override fun toString(): String {
         return name
     }