Rollup merge of #64457 - petrochenkov:macunfield, r=matthewjasper

def_collector: Do not ICE on attributes on unnamed fields

The primary issue here is that the expansion infra needs to visit a field in isolation, and fields don't know their own indices during expansion, so they have to be kept in some other place (e.g. `struct Definitions`).

Fixes https://github.com/rust-lang/rust/issues/64385

src/test/ui/attributes/unnamed-field-attributes.rs

@@ -0,0 +1,9 @@
+// check-pass
+
+struct S(
+    #[rustfmt::skip] u8,
+    u16,
+    #[rustfmt::skip] u32,
+);
+
+fn main() {}