Remove 'public' from a few NSError-bridging-related symbols (#14731)

We're still using most of this, but not all of it has to be public.

rdar://problem/35230025

Author: jrose-apple

stdlib/public/SDK/Foundation/NSError.swift

@@ -24,8 +24,16 @@ public typealias NSErrorPointer = AutoreleasingUnsafeMutablePointer<NSError?>?
 // Note: NSErrorPointer becomes ErrorPointer in Swift 3.
 public typealias ErrorPointer = NSErrorPointer
 
-public // COMPILER_INTRINSIC
-let _nilObjCError: Error = _GenericObjCError.nilError
+// An error value to use when an Objective-C API indicates error
+// but produces a nil error object.
+// This is 'internal' rather than 'private' for no other reason but to make the
+// type print more nicely. It's not part of the ABI, so if type printing of
+// private things improves we can change it.
+internal enum _GenericObjCError : Error {
+  case nilError
+}
+// A cached instance of the above in order to save on the conversion to Error.
+private let _nilObjCError: Error = _GenericObjCError.nilError
 
 public // COMPILER_INTRINSIC
 func _convertNSErrorToError(_ error: NSError?) -> Error {
@@ -335,12 +343,6 @@ extension CFError : Error {
   }
 }
 
-// An error value to use when an Objective-C API indicates error
-// but produces a nil error object.
-public enum _GenericObjCError : Error {
-  case nilError
-}
-
 /// An internal protocol to represent Swift error enums that map to standard
 /// Cocoa NSError domains.
 public protocol _ObjectiveCBridgeableError : Error {

test/Inputs/clang-importer-sdk/swift-modules-without-ns/Foundation.swift

@@ -169,7 +169,7 @@ extension NSError : Error {
   public var _code: Int { return code }
 }
 
-public enum _GenericObjCError : Error {
+internal enum _GenericObjCError : Error {
   case nilError
 }
 

test/Inputs/clang-importer-sdk/swift-modules/Foundation.swift

@@ -230,7 +230,7 @@ extension NSError : Error {
   public var _code: Int { return code }
 }
 
-public enum _GenericObjCError : Error {
+internal enum _GenericObjCError : Error {
   case nilError
 }