Make #[debug_format] an attribute in newtype_index

This removes the `custom` format functionality as its only user was
trivially migrated to using a normal format.

If a new use case for a custom formatting impl pops up, you can add it
back.

compiler/rustc_passes/src/liveness.rs

@@ -108,14 +108,14 @@ use std::rc::Rc;
 mod rwu_table;
 
 rustc_index::newtype_index! {
+    #[debug_format = "v({})"]
     pub struct Variable {
-        DEBUG_FORMAT = "v({})",
     }
 }
 
 rustc_index::newtype_index! {
+    #[debug_format = "ln({})"]
     pub struct LiveNode {
-        DEBUG_FORMAT = "ln({})",
     }
 }