Rollup merge of #130607 - GnomedDev:remove-seekfrom-paths, r=compiler-errors

[Clippy] Remove final std paths for diagnostic item

Removes the paths to SeekFrom::Start/Current that were left in #130553.

This was split off as it involves introducing a utility to check for enum ctors, as both:
- enum variants cannot be diagnostic items
- even if they could, that wouldn't help because we need to get the enum variant ctor

While adding the `is_enum_variant_ctor`, I removed both `is_diagnostic_ctor` and `is_res_diagnostic_ctor` as they are unused and never worked due to the above bullet points.

compiler/rustc_span/src/symbol.rs

@@ -309,6 +309,7 @@ symbols! {
         RwLockReadGuard,
         RwLockWriteGuard,
         Saturating,
+        SeekFrom,
         Send,
         SeqCst,
         Sized,