[utils] Move atMostOne to core/util.runtime

There is nothing backend-specific about this helper function.

compiler/backend-common/src/org/jetbrains/kotlin/backend/common/CollectionUtil.kt

@@ -1,16 +0,0 @@
-/*
- * Copyright 2000-2019 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.backend.common
-
-fun <T> Collection<T>.atMostOne(): T? {
-    return when (this.size) {
-        0 -> null
-        1 -> this.iterator().next()
-        else -> throw IllegalArgumentException("Collection has more than one element.")
-    }
-}
-
-inline fun <T> Iterable<T>.atMostOne(predicate: (T) -> Boolean): T? = this.filter(predicate).atMostOne()

compiler/backend-common/src/org/jetbrains/kotlin/backend/common/SimpleMemberScope.kt

@@ -11,6 +11,7 @@ import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.resolve.scopes.DescriptorKindFilter
 import org.jetbrains.kotlin.resolve.scopes.MemberScopeImpl
 import org.jetbrains.kotlin.utils.Printer
+import org.jetbrains.kotlin.utils.atMostOne
 
 class SimpleMemberScope(val members: List<DeclarationDescriptor>) : MemberScopeImpl() {