stdlib: add an unavailable ClosedRange.clamp() function (#2908)

Author: gribozavr

stdlib/public/core/Range.swift.gyb

@@ -614,6 +614,24 @@ extension Range {
   }
 }
 
+extension ClosedRange {
+  @available(*, unavailable, message: "Call clamped(to:) and swap the argument and the receiver.  For example, x.clamp(y) becomes y.clamped(to: x) in Swift 3.")
+  public func clamp(
+    _ intervalToClamp: ClosedRange<Bound>
+  ) -> ClosedRange<Bound> {
+    Builtin.unreachable()
+  }
+}
+
+extension CountableClosedRange {
+  @available(*, unavailable, message: "Call clamped(to:) and swap the argument and the receiver.  For example, x.clamp(y) becomes y.clamped(to: x) in Swift 3.")
+  public func clamp(
+    _ intervalToClamp: CountableClosedRange<Bound>
+  ) -> CountableClosedRange<Bound> {
+    Builtin.unreachable()
+  }
+}
+
 @available(*, unavailable, message: "IntervalType has been removed in Swift 3. Use ranges instead.")
 public typealias IntervalType = Void