Rollup merge of #104593 - compiler-errors:rpitit-object-safety-spans, r=fee1-dead

Improve spans for RPITIT object-safety errors

No reason why we can't point at the `impl Trait` that causes the object-safety violation.

Also [drive-by: Add is_async fn to hir::IsAsync](https://github.com/rust-lang/rust/pull/104593/commits/c4165f3a965e258531928180195637455299c6f3), which touches clippy too.

compiler/rustc_hir/src/hir.rs

@@ -2720,6 +2720,12 @@ pub enum IsAsync {
     NotAsync,
 }
 
+impl IsAsync {
+    pub fn is_async(self) -> bool {
+        self == IsAsync::Async
+    }
+}
+
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, Encodable, Decodable, HashStable_Generic)]
 pub enum Defaultness {
     Default { has_value: bool },