[FIR LT] Extract getChildren function into common utils

compiler/frontend.common/src/org/jetbrains/kotlin/util/LightTreeUtils.kt

@@ -0,0 +1,16 @@
+/*
+ * 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.util
+
+import com.intellij.lang.LighterASTNode
+import com.intellij.openapi.util.Ref
+import com.intellij.util.diff.FlyweightCapableTreeStructure
+
+fun LighterASTNode.getChildren(tree: FlyweightCapableTreeStructure<LighterASTNode>): List<LighterASTNode> {
+    val children = Ref<Array<LighterASTNode?>>()
+    val count = tree.getChildren(this, children)
+    return if (count > 0) children.get().filterNotNull() else emptyList()
+}