avoid clone path prefix when lowering to hir Found this while trying to parallelize `lower_to_hir`. When lowering to hir, `Nested` paths in `ast` will be split and the prefix segments will be cloned. This could be omited, since the only consequence is that the prefix segments in `Path`s in hir will have the same `HirId`s, and it seems harmless. This simplifies the process of lowering to hir and avoids re-modification of `ResolverAstLowering`. r? `@Aaron1011` cc #99292