Stabilize RFC3324 dyn upcasting coercion

Aka trait_upcasting feature.

And also adjust the `deref_into_dyn_supertrait` lint.

src/lib.rs

@@ -11,7 +11,7 @@
 #![feature(round_ties_even)]
 #![feature(let_chains)]
 #![feature(lint_reasons)]
-#![feature(trait_upcasting)]
+#![cfg_attr(bootstrap, feature(trait_upcasting))]
 // Configure clippy and other lints
 #![allow(
     clippy::collapsible_else_if,

tests/fail/dyn-upcast-trait-mismatch.rs

@@ -1,6 +1,3 @@
-#![feature(trait_upcasting)]
-#![allow(incomplete_features)]
-
 trait Foo: PartialEq<i32> + std::fmt::Debug + Send + Sync {
     fn a(&self) -> i32 {
         10

tests/pass/box-custom-alloc.rs

@@ -1,7 +1,6 @@
 //@revisions: stack tree
 //@[tree]compile-flags: -Zmiri-tree-borrows
-#![allow(incomplete_features)] // for trait upcasting
-#![feature(allocator_api, trait_upcasting)]
+#![feature(allocator_api)]
 
 use std::alloc::Layout;
 use std::alloc::{AllocError, Allocator};

tests/pass/dyn-upcast.rs

@@ -1,6 +1,3 @@
-#![feature(trait_upcasting)]
-#![allow(incomplete_features)]
-
 fn main() {
     basic();
     diamond();