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

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

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

Related: #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;