Fix VecMap::iter_mut

It used to allow you to mutate the key, even though that can invalidate the map by creating duplicate keys.

compiler/rustc_data_structures/src/lib.rs

@@ -24,6 +24,7 @@
 #![feature(new_uninit)]
 #![feature(once_cell)]
 #![feature(maybe_uninit_uninit_array)]
+#![feature(min_type_alias_impl_trait)]
 #![allow(rustc::default_hash_types)]
 #![deny(unaligned_references)]