Fix <BoundConstness as Display>

There was infinite recursion, which is not very good. I'm not sure what
the best way to implement this is, I just did something that felt right.

compiler/rustc_middle/src/ty/mod.rs

@@ -333,7 +333,8 @@ impl fmt::Display for BoundConstness {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
             Self::NotConst => f.write_str("normal"),
-            _ => write!(f, "`{self}`"),
+            Self::Const => f.write_str("const"),
+            Self::ConstIfConst => f.write_str("~const"),
         }
     }
 }