compiler: Fix a couple issues around cargo feature unification

The first was a warning:
```
Testing stage2 {rustc_parse_format} (aarch64-apple-darwin)
   Compiling rustc_index v0.0.0 (/Users/ci/project/compiler/rustc_index)
error: extern crate `smallvec` is unused in crate `rustc_index`
 --> compiler/rustc_index/src/lib.rs:2:1
  |
2 | #![cfg_attr(all(feature = "nightly", test), feature(stmt_expr_attributes))]
  | ^
  |
  = help: remove the dependency or add `use smallvec as _;` to the crate root
  = note: `-D unused-crate-dependencies` implied by `-D warnings`
  = help: to override `-D warnings` add `#[allow(unused_crate_dependencies)]`

error: could not compile `rustc_index` (lib) due to 1 previous error
```

The second was that `type_ir_macros` didn't fully specify its
dependencies:
```
Testing stage1 {rustc_type_ir_macros} (aarch64-apple-darwin)
   Compiling rustc_type_ir_macros v0.0.0 (/Users/jyn/src/ferrocene3/compiler/rustc_type_ir_macros)
error[E0432]: unresolved import `syn::visit_mut`
   --> compiler/rustc_type_ir_macros/src/lib.rs:2:10
    |
  2 | use syn::visit_mut::VisitMut;
    |          ^^^^^^^^^ could not find `visit_mut` in `syn`
    |
note: found an item that was configured out
   --> /Users/jyn/.local/lib/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/syn-2.0.106/src/lib.rs:880:21
    |
878 | #[cfg(feature = "visit-mut")]
    |       --------------------- the item is gated behind the `visit-mut` feature
879 | #[cfg_attr(docsrs, doc(cfg(feature = "visit-mut")))]
880 | pub use crate::gen::visit_mut;
    |                     ^^^^^^^^^

error[E0433]: failed to resolve: could not find `visit_mut` in `syn`
   --> compiler/rustc_type_ir_macros/src/lib.rs:206:18
    |
206 |             syn::visit_mut::visit_type_path_mut(self, i);
    |                  ^^^^^^^^^ could not find `visit_mut` in `syn`
    |
note: found an item that was configured out
   --> /Users/jyn/.local/lib/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/syn-2.0.106/src/lib.rs:880:21
    |
878 | #[cfg(feature = "visit-mut")]
    |       --------------------- the item is gated behind the `visit-mut` feature
879 | #[cfg_attr(docsrs, doc(cfg(feature = "visit-mut")))]
880 | pub use crate::gen::visit_mut;
    |                     ^^^^^^^^^
```