Add 'suspendWithCurrentContinuation' and 'Suspend' object in built-ins

They are part of the new suspension convention, relevant
support in backends will be added in later commits

 #KT-14924 In Progress

core/builtins/native/kotlin/Coroutines.kt

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2010-2016 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.coroutines
+
+/**
+ * This function allows to obtain a continuation instance inside the suspend functions.
+ * As a suspend function may be only tail-called inside another suspend function, this call will be used as a return value of the latter one.
+ *
+ * If the [body] returns the [Suspend] object, that means that suspend-function cannot return a value immediately,
+ * i.e. it's literally suspends continuation, and the continuation will be resumed at some moment in the future.
+ *
+ * Otherwise return value must have a type assignable to 'T'.
+ * As the latter part cannot be correctly typechecked, it remains on the conscience of the suspend function's author.
+ *
+ * Note that it's not recommended to call a [Continuation] method in the same stackframe where suspension function is run,
+ * they should be called asynchronously later (probably from the different thread).
+ *
+ */
+@SinceKotlin("1.1")
+public inline suspend fun <T> suspendWithCurrentContinuation(body: (Continuation<T>) -> Any?): T