[FIR] Cleanup caching symbol in ConeClassLikeLookupTagImpl

Type of `boundSymbol` replaced to OneElementWeakMap
`FirSymbolProvider.getSymbolByLookupTag` moved to extensions

compiler/fir/tree/src/org/jetbrains/kotlin/fir/symbols/impl/ConeClassLikeLookupTagImpl.kt

@@ -5,11 +5,16 @@
 
 package org.jetbrains.kotlin.fir.symbols.impl
 
+import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.symbols.ConeClassLikeLookupTag
 import org.jetbrains.kotlin.name.ClassId
 
+@RequiresOptIn
+annotation class LookupTagInternals
+
 class ConeClassLikeLookupTagImpl(override val classId: ClassId) : ConeClassLikeLookupTag() {
-    var boundSymbol: Pair<*, FirClassLikeSymbol<*>?>? = null
+    @LookupTagInternals
+    var boundSymbol: OneElementWeakMap<FirSession, FirClassLikeSymbol<*>?>? = null
 
     override fun equals(other: Any?): Boolean {
         if (this === other) return true
@@ -25,4 +30,4 @@ class ConeClassLikeLookupTagImpl(override val classId: ClassId) : ConeClassLikeL
     override fun hashCode(): Int {
         return classId.hashCode()
     }
-}
+}

compiler/fir/tree/src/org/jetbrains/kotlin/fir/symbols/impl/OneElementWeakMap.kt

@@ -0,0 +1,21 @@
+/*
+ * Copyright 2010-2020 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.symbols.impl
+
+import java.lang.ref.WeakReference
+
+class OneElementWeakMap<K, V> private constructor(
+    private val keyReference: WeakReference<K>,
+    private val valueReference: WeakReference<V>
+) {
+    constructor(key: K, value: V) : this(WeakReference(key), WeakReference(value))
+
+    val key: K?
+        get() = keyReference.get()
+
+    val value: V?
+        get() = valueReference.get()
+}