FIR: add bfs helper and use it

It's also used in Kotlin quickfixes in intellij repo

core/compiler.common/src/org/jetbrains/kotlin/util/containerUtils.kt

@@ -0,0 +1,23 @@
+/*
+ * Copyright 2010-2021 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
+
+/**
+ * Perform BFS on the given collection with neighbors created by the given function.
+ */
+fun <T> Collection<T>.bfs(getNeighbors: (T) -> Iterator<T>): Sequence<T> {
+    val queue = ArrayDeque(this)
+    val visited = mutableSetOf<T>()
+    return sequence {
+        while (queue.isNotEmpty()) {
+            val current = queue.removeFirst()
+            if (current in visited) continue
+            visited.add(current)
+            yield(current)
+            getNeighbors(current).forEach(queue::add)
+        }
+    }
+}