Allow unused fields in some tests

The dead_code lint was previously eroneously missing those.
Since this lint bug has been fixed, the unused fields warnings need
to be fixed.

Most of them are marked as `#[allow(dead_code)]`. Other warnings are
fixed by changing visibility of modules.

tests/ui/impl-not-adjacent-to-type.rs

@@ -3,6 +3,7 @@
 mod foo {
     pub struct Point {
         pub x: i32,
+        #[allow(dead_code)]
         pub y: i32,
     }
 }

tests/ui/privacy/suggest-making-field-public.fixed

@@ -1,5 +1,5 @@
 //@ run-rustfix
-mod a {
+pub mod a {
     pub struct A(pub String);
 }
 

tests/ui/privacy/suggest-making-field-public.rs

@@ -1,5 +1,5 @@
 //@ run-rustfix
-mod a {
+pub mod a {
     pub struct A(pub(self)String);
 }
 

tests/ui/union/union-macro.rs

@@ -15,6 +15,7 @@ mod m2 {
 
 duplicate! {
     pub union U {
+        #[allow(dead_code)]
         pub a: u8
     }
 }