Convert newtype_index to a proc macro

The `macro_rules!` implementation was becomng excessively complicated,
and difficult to modify. The new proc macro implementation should make
it much easier to add new features (e.g. skipping certain `#[derive]`s)

compiler/rustc_index/src/lib.rs

@@ -12,6 +12,8 @@ pub mod bit_set;
 pub mod interval;
 pub mod vec;
 
+pub use rustc_macros::newtype_index;
+
 /// Type size assertion. The first argument is a type and the second argument is its expected size.
 #[macro_export]
 macro_rules! static_assert_size {