Add PurelyImplements annotation

It's parameter is FQ-name of class (currently only from builtins) that added as supertype to annotated Java class.

Parameters of annotated class used as non-flexible arguments of added supertype, that helps to propagate more precise types when using in Kotlin.
Some standard JDK collections loaded as they annotated with PurelyImplements.

See tests for clarification.
Before: ArrayList<Int>.add(x: Int!) // possible to add null
After: ArrayList<Int>.add(x: Int)   // impossible to add null

 #KT-7628 Fixed
 #KT-7835 Fixed

core/runtime.jvm/src/kotlin/jvm/PurelyImplements.kt

@@ -0,0 +1,37 @@
+/*
+ * 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 kotlin.jvm
+
+/**
+ * Instructs the Kotlin compiler to treat annotated Java class as pure implementation of given Kotlin interface.
+ * "Pure" means here that each type parameter of class becomes non-platform type argument of that interface.
+ *
+ * Example:
+ *
+ * class MyList<T> extends AbstractList<T> { ... }
+ *
+ * Methods defined in MyList<T> use T as platform, i.e. it's possible to perform unsafe operation in Kotlin:
+ *  MyList<Int>().add(null) // compiles
+ *
+ * @PurelyImplements("kotlin.MutableList")
+ * class MyPureList<T> extends AbstractList<T> { ... }
+ *
+ * Methods defined in MyPureList<T> overriding methods in MutableList use T as non-platform types:
+ *  MyList<Int>().add(null) // Error
+ *  MyList<Int?>().add(null) // Ok
+ */
+public annotation class PurelyImplements(public val value: String)