[FIR] Store cachedExpandedType in WeakPair instead of usual Pair

WeakPair is a renamed OneElementWeakMap

compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/impl/Impl.kt

@@ -10,6 +10,7 @@ import org.jetbrains.kotlin.fir.types.ConeAttributes
 import org.jetbrains.kotlin.fir.types.ConeClassLikeType
 import org.jetbrains.kotlin.fir.types.ConeTypeProjection
 import org.jetbrains.kotlin.fir.types.ConeNullability
+import org.jetbrains.kotlin.fir.utils.WeakPair
 
 class ConeClassLikeTypeImpl(
     override val lookupTag: ConeClassLikeLookupTag,
@@ -20,7 +21,7 @@ class ConeClassLikeTypeImpl(
     override val nullability: ConeNullability = ConeNullability.create(isNullable)
 
     // Cached expanded type and the relevant session
-    var cachedExpandedType: Pair<*, ConeClassLikeType>? = null
+    var cachedExpandedType: WeakPair<*, ConeClassLikeType>? = null
 
     override fun equals(other: Any?): Boolean {
         if (this === other) return true

compiler/fir/cones/src/org/jetbrains/kotlin/fir/utils/WeakPair.kt

@@ -0,0 +1,27 @@
+/*
+ * 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.fir.utils
+
+import java.lang.ref.WeakReference
+
+class WeakPair<K, V>(first: K, second: V) {
+    private val firstReference: WeakReference<K> = WeakReference(first)
+    private val secondReference: WeakReference<V> = WeakReference(second)
+
+    val first: K?
+        get() = firstReference.get()
+
+    val second: V?
+        get() = secondReference.get()
+}
+
+operator fun <K, V> WeakPair<K, V>?.component1(): K? {
+    return this?.first
+}
+
+operator fun <K, V> WeakPair<K, V>?.component2(): V? {
+    return this?.second
+}