Let codegen decide when to mem::swap with immediates

Making `libcore` decide this is silly; the backend has so much better information about when it's a good idea.

So introduce a new `typed_swap` intrinsic with a fallback body, but replace that implementation for immediates and scalar pairs.

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -500,6 +500,8 @@ pub fn check_intrinsic_type(
                 (1, 0, vec![Ty::new_mut_ptr(tcx, param(0)), param(0)], Ty::new_unit(tcx))
             }
 
+            sym::typed_swap => (1, 1, vec![Ty::new_mut_ptr(tcx, param(0)); 2], Ty::new_unit(tcx)),
+
             sym::discriminant_value => {
                 let assoc_items = tcx.associated_item_def_ids(
                     tcx.require_lang_item(hir::LangItem::DiscriminantKind, None),