runtime: add a workaround for Windows build

The runtime tests will statically link the runtime and dynamically link
to the standard library.  This fails to build on Windows.  This is a
horrible workaround for the time being.

Author: compnerd

stdlib/public/runtime/MetadataLookup.cpp

@@ -730,8 +730,14 @@ _findContextDescriptorInCache(TypeMetadataPrivateState &T,
   $sS ## CHAR ## SUFFIX
 #define DESCRIPTOR_MANGLING(CHAR, SUFFIX) DESCRIPTOR_MANGLING_(CHAR, SUFFIX)
 
+#if defined(_WIN32)
+#define swiftCore_EXPORTS __declspec(dllimport)
+#else
+#define swiftCore_EXPORTS
+#endif
+
 #define STANDARD_TYPE(KIND, MANGLING, TYPENAME) \
-  extern "C" const ContextDescriptor DESCRIPTOR_MANGLING(MANGLING, DESCRIPTOR_MANGLING_SUFFIX(KIND));
+  extern "C" swiftCore_EXPORTS const ContextDescriptor DESCRIPTOR_MANGLING(MANGLING, DESCRIPTOR_MANGLING_SUFFIX(KIND));
 
 #if !SWIFT_OBJC_INTEROP
 # define OBJC_INTEROP_STANDARD_TYPE(KIND, MANGLING, TYPENAME)