[FIR generator] Factor out TypeArgument to common module

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/ArbitrarilyImportable.kt

@@ -5,4 +5,6 @@
 
 package org.jetbrains.kotlin.generators.tree
 
-data class ArbitraryImportable(override val packageName: String, override val type: String) : Importable
+data class ArbitraryImportable(override val packageName: String, override val type: String) : Importable {
+    override fun getTypeWithArguments(notNull: Boolean): String = type
+}

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

@@ -9,4 +9,8 @@ interface Importable {
     val type: String
     val packageName: String?
     val fullQualifiedName: String? get() = packageName?.let { "$it.${type.replace(Regex("<.+>"), "")}" }
-}
+
+    fun getTypeWithArguments(notNull: Boolean = false): String
+}
+
+val Importable.typeWithArguments: String get() = getTypeWithArguments()

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

@@ -0,0 +1,28 @@
+/*
+ * 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
+
+sealed class TypeArgument(val name: String) {
+    abstract val upperBounds: List<Importable>
+}
+
+class SimpleTypeArgument(name: String, val upperBound: Importable?) : TypeArgument(name) {
+    override val upperBounds: List<Importable> = listOfNotNull(upperBound)
+
+    override fun toString(): String {
+        var result = name
+        if (upperBound != null) {
+            result += " : ${upperBound.typeWithArguments}"
+        }
+        return result
+    }
+}
+
+class TypeArgumentWithMultipleUpperBounds(name: String, override val upperBounds: List<Importable>) : TypeArgument(name) {
+    override fun toString(): String {
+        return name
+    }
+}