Unify region variables when projecting associated types

This is required to avoid cycles when evaluating auto trait
predicates.

src/test/ui/issues/issue-23080-2.rs

@@ -1,15 +0,0 @@
-//~ ERROR
-
-#![feature(optin_builtin_traits)]
-#![feature(negative_impls)]
-
-unsafe auto trait Trait {
-    type Output; //~ ERROR E0380
-}
-
-fn call_method<T: Trait>(x: T) {}
-
-fn main() {
-    // ICE
-    call_method(());
-}

src/test/ui/issues/issue-23080-2.stderr

@@ -1,17 +0,0 @@
-error[E0380]: auto traits cannot have methods or associated items
-  --> $DIR/issue-23080-2.rs:7:10
-   |
-LL | unsafe auto trait Trait {
-   |                   ----- auto trait cannot have items
-LL |     type Output;
-   |          ^^^^^^
-
-error[E0275]: overflow evaluating the requirement `<() as Trait>::Output`
-   |
-   = note: required because of the requirements on the impl of `Trait` for `()`
-   = note: required because of the requirements on the impl of `Trait` for `()`
-
-error: aborting due to 2 previous errors
-
-Some errors have detailed explanations: E0275, E0380.
-For more information about an error, try `rustc --explain E0275`.

src/test/ui/issues/issue-23080.rs

@@ -1,17 +0,0 @@
-#![feature(optin_builtin_traits)]
-#![feature(negative_impls)]
-
-unsafe auto trait Trait {
-    fn method(&self) { //~ ERROR E0380
-        println!("Hello");
-    }
-}
-
-fn call_method<T: Trait>(x: T) {
-    x.method();
-}
-
-fn main() {
-    // ICE
-    call_method(());
-}

src/test/ui/issues/issue-23080.stderr

@@ -1,11 +0,0 @@
-error[E0380]: auto traits cannot have methods or associated items
-  --> $DIR/issue-23080.rs:5:8
-   |
-LL | unsafe auto trait Trait {
-   |                   ----- auto trait cannot have items
-LL |     fn method(&self) {
-   |        ^^^^^^
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0380`.