Auto merge of rust-lang#81498 - thomaseizinger:ice-workaround-56935-rustc-index, r=matthewjasper

Apply workaround from rust-lang#72003 for rust-lang#56935 to allow for cross-compilation of `rustc_index` crate

This patch applies the same workaround as rust-lang#72003 to the `rustc_index` crate. This allows recent versions of rustfmt to compile to wasm again.

Related: rust-lang#72017.

Author: bors

compiler/rustc_index/src/lib.rs

@@ -8,3 +8,7 @@
 
 pub mod bit_set;
 pub mod vec;
+
+// FIXME(#56935): Work around ICEs during cross-compilation.
+#[allow(unused)]
+extern crate rustc_macros;