[Runtime] Fix debugDescription of .self keypaths.

AnyKeyPath's debugDescription assumes there's always at least one component, but `\Type.self` produces an empty keypath. Special-case the empty case to display a `.self` component.

rdar://103237845

stdlib/public/core/KeyPath.swift

@@ -3934,7 +3934,8 @@ extension AnyKeyPath: CustomDebugStringConvertible {
     return withBuffer {
       var buffer = $0
       if buffer.data.isEmpty {
-        _internalInvariantFailure("key path has no components")
+        description.append(".self")
+        return description
       }
       var valueType: Any.Type = Self.rootType
       while true {