Don't install default projection bound for RPITITs

tests/ui/impl-trait/in-trait/check-wf-on-non-defaulted-rpitit.rs

@@ -0,0 +1,10 @@
+#![feature(return_position_impl_trait_in_trait)]
+
+struct Wrapper<G: Send>(G);
+
+trait Foo {
+    fn bar() -> Wrapper<impl Sized>;
+    //~^ ERROR `impl Sized` cannot be sent between threads safely
+}
+
+fn main() {}

tests/ui/impl-trait/in-trait/check-wf-on-non-defaulted-rpitit.stderr

@@ -0,0 +1,16 @@
+error[E0277]: `impl Sized` cannot be sent between threads safely
+  --> $DIR/check-wf-on-non-defaulted-rpitit.rs:6:17
+   |
+LL |     fn bar() -> Wrapper<impl Sized>;
+   |                 ^^^^^^^^^^^^^^^^^^^ `impl Sized` cannot be sent between threads safely
+   |
+   = help: the trait `Send` is not implemented for `impl Sized`
+note: required by a bound in `Wrapper`
+  --> $DIR/check-wf-on-non-defaulted-rpitit.rs:3:19
+   |
+LL | struct Wrapper<G: Send>(G);
+   |                   ^^^^ required by this bound in `Wrapper`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.