[FIR] Refactoring: Move set utility functions to setUtils.kt

compiler/util/src/org/jetbrains/kotlin/utils/setUtils.kt

@@ -0,0 +1,25 @@
+/*
+ * 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.utils
+
+/**
+ * Works almost as regular flatMap, but returns a set and returns null if any lambda call returned null
+ */
+inline fun <T, R> Iterable<T>.flatMapToNullableSet(transform: (T) -> Iterable<R>?): Set<R>? =
+    flatMapTo(mutableSetOf()) { transform(it) ?: return null }.ifEmpty { emptySet() }
+
+/**
+ * Maps all elements of this non-empty collection with the given [transform] function to a new mutable set, or returns [emptySet] if this
+ * collection is empty.
+ *
+ * [mapToSetOrEmpty] should be preferred over `collection.mapTo(mutableSetOf()) { ... }` when `collection` may be empty and the resulting
+ * set may be cached, because [mapToSetOrEmpty] saves memory by avoiding the creation of an empty mutable set.
+ */
+inline fun <T, R> Collection<T>.mapToSetOrEmpty(transform: (T) -> R): Set<R> =
+    if (isNotEmpty()) mapTo(mutableSetOf(), transform) else emptySet()
+
+inline fun <T> Collection<T>.filterToSetOrEmpty(predicate: (T) -> Boolean): Set<T> =
+    filterTo(mutableSetOf(), predicate).ifEmpty { emptySet() }