[FIR] Move ScopeSession to fir:tree

compiler/fir/tree/src/org/jetbrains/kotlin/fir/resolve/ScopeSession.kt

@@ -0,0 +1,22 @@
+/*
+ * Copyright 2010-2019 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.fir.resolve
+
+import org.jetbrains.kotlin.fir.scopes.FirScope
+
+class ScopeSession {
+    private val scopes = hashMapOf<Any, HashMap<ScopeSessionKey<*, *>, FirScope>>()
+    fun <ID : Any, FS : FirScope> getOrBuild(id: ID, key: ScopeSessionKey<ID, FS>, build: () -> FS): FS {
+        return scopes.getOrPut(id) {
+            hashMapOf()
+        }.getOrPut(key) {
+            build()
+        } as FS
+    }
+}
+
+abstract class ScopeSessionKey<ID : Any, FS : FirScope>
+