runtime: make _SwiftNativeNSError use the Hashable conformance, if available

If the Swift error wrapped in a _SwiftNativeNSError box conforms to
Hashable, the box now uses the Swift's conformance to Hashable.

Part of rdar://problem/27574348.

stdlib/public/runtime/SwiftHashableSupport.h

@@ -0,0 +1,53 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2016 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_RUNTIME_SWIFT_HASHABLE_SUPPORT_H
+#define SWIFT_RUNTIME_SWIFT_HASHABLE_SUPPORT_H
+
+#include "swift/Runtime/Metadata.h"
+#include <stdint.h>
+
+namespace swift {
+namespace hashable_support {
+
+/// The name demangles to "protocol descriptor for Swift.Hashable".
+extern "C" const ProtocolDescriptor _TMps8Hashable;
+
+struct HashableWitnessTable;
+
+/// Calls `Equatable.==` through a `Hashable` (not Equatable!) witness
+/// table.
+extern "C" bool swift_stdlib_Hashable_isEqual_indirect(
+    const void *lhsValue, const void *rhsValue, const Metadata *type,
+    const HashableWitnessTable *wt);
+
+/// Calls `Hashable.hashValue.get` through a `Hashable` witness table.
+extern "C" intptr_t swift_stdlib_Hashable_hashValue_indirect(
+    const void *value, const Metadata *type, const HashableWitnessTable *wt);
+
+/// Find the base type that introduces the `Hashable` conformance.
+/// Because the provided type is known to conform to `Hashable`, this
+/// function always returns non-null.
+///
+/// - Precondition: `type` conforms to `Hashable` (not checked).
+const Metadata *findHashableBaseTypeOfHashableType(
+    const Metadata *type);
+
+/// Find the base type that introduces the `Hashable` conformance.
+/// If `type` does not conform to `Hashable`, `nullptr` is returned.
+const Metadata *findHashableBaseType(const Metadata *type);
+
+} // namespace hashable_support
+} // namespace swift
+
+#endif
+