[AA] Add KotlinDirectInheritorsProvider

- Direct inheritors are needed to calculate sealed inheritors. The new
  `KotlinDirectInheritorsProvider` can be used to implement a common
  sealed inheritors provider in LL FIR.

^KT-66013

analysis/analysis-api-providers/src/org/jetbrains/kotlin/analysis/providers/KotlinDirectInheritorsProvider.kt

@@ -0,0 +1,32 @@
+/*
+ * Copyright 2010-2024 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.analysis.providers
+
+import com.intellij.openapi.project.Project
+import com.intellij.psi.search.GlobalSearchScope
+import org.jetbrains.kotlin.psi.KtClass
+import org.jetbrains.kotlin.psi.KtClassOrObject
+
+public interface KotlinDirectInheritorsProvider {
+    /**
+     * Returns all direct inheritors of [ktClass] that can be found in the given [scope]. If [includeLocalInheritors] is `false`, only
+     * non-local inheritors will be returned.
+     *
+     * The implementor of [getDirectKotlinInheritors] is allowed to lazy-resolve symbols up to the `SUPER_TYPES` phase. This is required to
+     * check subtyping for potential inheritors. Hence, if [getDirectKotlinInheritors] is invoked during lazy resolution, it requires a
+     * phase of `SEALED_CLASS_INHERITORS` or later.
+     */
+    public fun getDirectKotlinInheritors(
+        ktClass: KtClass,
+        scope: GlobalSearchScope,
+        includeLocalInheritors: Boolean
+    ): Iterable<KtClassOrObject>
+
+    public companion object {
+        public fun getInstance(project: Project): KotlinDirectInheritorsProvider =
+            project.getService(KotlinDirectInheritorsProvider::class.java)
+    }
+}