Auto merge of #82898 - oli-obk:tait_🧊, r=nikomatsakis

Add a `min_type_alias_impl_trait` feature gate

This new feature gate only permits type alias impl trait to be constrained by function and trait method return types. All other possible constraining sites like const/static types, closure return types and binding types are now forbidden and gated under the `type_alias_impl_trait` and `impl_trait_in_bindings` feature gates (which are both marked as incomplete, as they have various ways to ICE the compiler or cause query cycles where they shouldn't).

r? `@nikomatsakis`

This is best reviewed commit-by-commit

compiler/rustc_hir/src/hir.rs

@@ -2291,7 +2291,9 @@ pub enum OpaqueTyOrigin {
     AsyncFn,
     /// `let _: impl Trait = ...`
     Binding,
-    /// Impl trait in type aliases, consts, statics, bounds.
+    /// type aliases: `type Foo = impl Trait;`
+    TyAlias,
+    /// Impl trait consts, statics, bounds.
     Misc,
 }