Minor. Refine substitution for SAM adapters

Introduce CompositionTypeSubstitution instead of manual composition of
two substitutions.

This change is necessary for the later changes where original substitution
will be able to contain outer type parameters

 #KT-11128 In Progress

core/descriptors/src/org/jetbrains/kotlin/types/CompositionTypeSubstitution.kt

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2010-2015 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.jetbrains.kotlin.types
+
+import org.jetbrains.kotlin.descriptors.TypeParameterDescriptor
+
+// Composition works as it's expected: Composition(outer, inner)(type) = outer(inner(type))
+// Note that when a substitution returns null it means that type shouldn't be changed (i.e. it works like the identity function in that case)
+// Currently it's only works if the inner substitution is a mapping from type parameter to another type parameter.
+// Otherwise composition semantics become a little bit complicated.
+class CompositionTypeSubstitution(
+        private val outer: TypeSubstitution, private val inner: Map<TypeParameterDescriptor, TypeParameterDescriptor>
+) : DelegatedTypeSubstitution(outer) {
+
+    override fun get(key: KotlinType) = inner[key.constructor.declarationDescriptor]?.let { outer[it.defaultType] } ?: outer[key]
+}