SwiftDemangle: correct linkage for COFF targets

This adds the DLL storage attribute on COFF, and switches to protected
visibility on ELF for the interfaces as a small optimization.

include/swift/SwiftDemangle/Platform.h

@@ -0,0 +1,44 @@
+//===-- SwiftDemangle/Platform.h - SwiftDemangle Platform Decls -*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_DEMANGLE_PLATFORM_H
+#define SWIFT_DEMANGLE_PLATFORM_H
+
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
+#if defined(SwiftDemangle_EXPORTS)
+# if defined(__ELF__)
+#   define SWIFT_DEMANGLE_LINKAGE __attribute__((__visibility__("protected")))
+# elif defined(__MACH__)
+#   define SWIFT_DEMANGLE_LINKAGE __attribute__((__visibility__("default")))
+# else
+#   define SWIFT_DEMANGLE_LINKAGE __declspec(dllexport)
+# endif
+#else
+# if defined(__ELF__)
+#   define SWIFT_DEMANGLE_LINKAGE __attribute__((__visibility__("default")))
+# elif defined(__MACH__)
+#   define SWIFT_DEMANGLE_LINKAGE __attribute__((__visibility__("default")))
+# else
+#   define SWIFT_DEMANGLE_LINKAGE __declspec(dllimport)
+# endif
+#endif
+
+#if defined(__cplusplus)
+}
+#endif
+
+#endif
+
+