SwiftRemoteMirror: repair the windows build

This symbol is meant to be exposed to users of the SwiftRemoteMirror
library which requires that it is explicitly marked with the appropriate
DLL storage on Windows.  This should repair the Windows build.

Author: compnerd

include/swift/SwiftRemoteMirror/SwiftRemoteMirror.h

@@ -37,6 +37,7 @@
 extern "C" {
 #endif
 
+SWIFT_REMOTE_MIRROR_LINKAGE
 extern unsigned long long swift_reflection_classIsSwiftMask;
 
 /// Get the metadata version supported by the Remote Mirror library.

stdlib/public/SwiftRemoteMirror/SwiftRemoteMirror.cpp

@@ -10,8 +10,13 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "swift/SwiftRemoteMirror/Platform.h"
+
 #define SWIFT_CLASS_IS_SWIFT_MASK swift_reflection_classIsSwiftMask
-extern "C" unsigned long long swift_reflection_classIsSwiftMask = 2;
+extern "C" {
+SWIFT_REMOTE_MIRROR_LINKAGE
+unsigned long long swift_reflection_classIsSwiftMask = 2;
+}
 
 #include "swift/Reflection/ReflectionContext.h"
 #include "swift/Reflection/TypeLowering.h"