[ty] Share names in synthesized constructor parameters (#28398)
When synthesizing constructor parameters for dataclasses and named
tuples, we rebuild each unaliased field name from `&str`. Names longer
than 16 bytes therefore allocate again even though the field already
owns shared storage.
Clone the existing `Name` when no alias is present so the generated
parameter shares its allocation with the field. Aliased parameters
retain their existing names.
