Add warn-by-default lint for visibility on const _ declarations

Add a warn-by-default `unused_visibility` lint for visibility qualifiers
on `const _` declarations - e.g. `pub const _: () = ();`.
These have no effect.

Author: Jules-Bertholet

tests/ui-toml/absolute_paths/absolute_paths.no_short.stderr

@@ -71,10 +71,10 @@ LL |         impl<T: core::cmp::Eq> core::fmt::Display for X<T>
    |                                ^^^^^^^^^^^^^^^^^^
 
 error: consider bringing this path into scope with the `use` keyword
-  --> tests/ui-toml/absolute_paths/absolute_paths.rs:113:14
+  --> tests/ui-toml/absolute_paths/absolute_paths.rs:113:10
    |
-LL | pub const _: crate::S = {
-   |              ^^^^^^^^
+LL | const _: crate::S = {
+   |          ^^^^^^^^
 
 error: consider bringing this path into scope with the `use` keyword
   --> tests/ui-toml/absolute_paths/absolute_paths.rs:114:9

tests/ui-toml/absolute_paths/absolute_paths.rs

@@ -110,7 +110,7 @@ mod m1 {
 }
 
 //~[no_short]v absolute_paths
-pub const _: crate::S = {
+const _: crate::S = {
     let crate::S = m1::S; //~[no_short] absolute_paths
 
     crate::m1::S