[stdlib] Mark public classes as 'open'. (#3876)

Part of SE-0117.

Author: jrose-apple

stdlib/public/core/ManagedBuffer.swift

@@ -23,7 +23,7 @@ import SwiftShims
 /// any live elements in the `deinit` of a subclass.
 /// - Note: Subclasses must not have any stored properties; any storage
 ///   needed should be included in `Header`.
-public class ManagedBuffer<Header, Element> {
+open class ManagedBuffer<Header, Element> {
 
   /// Create a new instance of the most-derived class, calling
   /// `factory` on the partially-constructed object to generate

stdlib/public/core/Runtime.swift.gyb

@@ -495,13 +495,15 @@ class _SwiftNativeNSSet {}
 @objc @_swift_native_objc_runtime_base(_SwiftNativeNSEnumeratorBase)
 class _SwiftNativeNSEnumerator {}
 
+// FIXME(ABI): move into the Foundation overlay and remove 'open'
 @objc @_swift_native_objc_runtime_base(_SwiftNativeNSDataBase)
-public class _SwiftNativeNSData {
+open class _SwiftNativeNSData {
   public init() {}
 }
 
+// FIXME(ABI): move into the Foundation overlay and remove 'open'
 @objc @_swift_native_objc_runtime_base(_SwiftNativeNSCharacterSetBase)
-public class _SwiftNativeNSCharacterSet {
+open class _SwiftNativeNSCharacterSet {
   public init() {}
 }