rust: pin-init: internal: use span of Index for generated identifiers

Use the span of `Index` for generated identifiers, so diagnostics can point
to the span of the index.

Link: https://patch.msgid.link/20260916184131.435059-1-gary@garyguo.net
Signed-off-by: Gary Guo <gary@garyguo.net>

rust/pin-init/internal/src/util.rs

@@ -41,7 +41,7 @@ impl MemberExt for Member {
     fn as_ident(&self) -> Ident {
         match self {
             Member::Named(ident) => ident.clone(),
-            Member::Unnamed(Index { index, .. }) => format_ident!("_{index}"),
+            Member::Unnamed(Index { index, span }) => format_ident!("_{index}", span = *span),
         }
     }