Auto merge of #112988 - spastorino:new-rpitit-24, r=compiler-errors

Replace RPITIT current impl with new strategy that lowers as a GAT

This PR replaces the current implementation of RPITITs with the new implementation that we had under -Zlower-impl-trait-in-trait-to-assoc-ty flag that lowers the RPIT as a GAT on the trait and on the impls that implement that trait.

Opening this PR as a draft because this goes after #112682, ~#112981~ and ~#112983~.
As soon as those are merged, I can rebase and we should run perf, crater and test a lot.

r? `@compiler-errors`

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -949,7 +949,6 @@ impl<'a, 'b, 'tcx> BuildReducedGraphVisitor<'a, 'b, 'tcx> {
                 | DefKind::TyAlias
                 | DefKind::ForeignTy
                 | DefKind::OpaqueTy
-                | DefKind::ImplTraitPlaceholder
                 | DefKind::TraitAlias
                 | DefKind::AssocTy,
                 _,