Adopt let_else across the compiler

This performs a substitution of code following the pattern:

let <id> = if let <pat> = ... { identity } else { ... : ! };

To simplify it to:

let <pat> = ... { identity } else { ... : ! };

By adopting the let_else feature.

compiler/rustc_trait_selection/src/traits/on_unimplemented.rs

@@ -164,9 +164,7 @@ impl<'tcx> OnUnimplementedDirective {
     ) -> Result<Option<Self>, ErrorReported> {
         let attrs = tcx.get_attrs(impl_def_id);
 
-        let attr = if let Some(item) = tcx.sess.find_by_name(&attrs, sym::rustc_on_unimplemented) {
-            item
-        } else {
+        let Some(attr) = tcx.sess.find_by_name(&attrs, sym::rustc_on_unimplemented) else {
             return Ok(None);
         };