[Runtime] Cast to the correct type.

Effectively NFC, because the compiler is not likely to ever notice the
aliasing violation here.

Author: DougGregor

include/swift/Runtime/Metadata.h

@@ -727,6 +727,7 @@ template <typename Runtime> struct TargetGenericMetadata;
 template <typename Runtime> struct TargetClassMetadata;
 template <typename Runtime> struct TargetStructMetadata;
 template <typename Runtime> struct TargetOpaqueMetadata;
+template <typename Runtime> struct TargetValueMetadata;
 
 // FIXME: https://bugs.swift.org/browse/SR-1155
 #pragma clang diagnostic push
@@ -902,7 +903,7 @@ struct TargetMetadata {
     case MetadataKind::Struct:
     case MetadataKind::Enum:
     case MetadataKind::Optional:
-      return static_cast<const TargetStructMetadata<Runtime> *>(this)
+      return static_cast<const TargetValueMetadata<Runtime> *>(this)
           ->Description;
     case MetadataKind::ForeignClass:
     case MetadataKind::Opaque: