Implement RFC 1268

This patch allows overlap to occur between any two impls of a trait for
traits which have no associated items.

Several compile-fail tests around coherence had to be changed to add at
least one item to the trait they test against.

Ref #29864

src/test/compile-fail/coherence-default-trait-impl.rs

@@ -10,7 +10,7 @@
 
 #![feature(optin_builtin_traits)]
 
-trait MyTrait {}
+trait MyTrait { fn foo() {} }
 
 impl MyTrait for .. {}
 //~^ ERROR redundant default implementations of trait `MyTrait`