Disallow combining LegacyWindowAlias and LegacyNamespace

This doesn't currently happen, and we don't expect it to happen in the future.

Fixes #645.

Author: Ms2ger

index.bs

@@ -9072,6 +9072,9 @@ and must not be one of the [=reserved identifiers=].
 The [{{LegacyWindowAlias}}] and [{{NoInterfaceObject}}]
 extended attributes must not be specified on the same interface.
 
+The [{{LegacyWindowAlias}}] and [{{LegacyNamespace}}]
+extended attributes must not be specified on the same interface.
+
 The [{{LegacyWindowAlias}}] extended attribute must not be specified
 on an interface that does not include the {{Window}} [=interface=]
 in its [=exposure set=].