Stop accepting 'impl ...;', require {} instead

Progress on #7981

src/test/debug-info/generic-trait-generic-static-default-method.rs

@@ -40,7 +40,7 @@ trait Trait<T1> {
     }
 }
 
-impl<T> Trait<T> for Struct;
+impl<T> Trait<T> for Struct {}
 
 fn main() {
 

src/test/debug-info/self-in-default-method.rs

@@ -118,7 +118,7 @@ trait Trait {
     }
 }
 
-impl Trait for Struct;
+impl Trait for Struct {}
 
 fn main() {
     let stack = Struct { x: 100 };

src/test/debug-info/self-in-generic-default-method.rs

@@ -119,7 +119,7 @@ trait Trait {
     }
 }
 
-impl Trait for Struct;
+impl Trait for Struct {}
 
 fn main() {
     let stack = Struct { x: 987 };

src/test/debug-info/trait-generic-static-default-method.rs

@@ -40,7 +40,7 @@ trait Trait {
     }
 }
 
-impl Trait for Struct;
+impl Trait for Struct {}
 
 fn main() {