Auto merge of #119192 - michaelwoerister:mcp533-push, r=cjgillot

Replace a number of FxHashMaps/Sets with stable-iteration-order alternatives

This PR replaces almost all of the remaining `FxHashMap`s in query results with either `FxIndexMap` or `UnordMap`. The only case that is missing is the `EffectiveVisibilities` struct which turned out to not be straightforward to transform. Once that is done too, we can remove the `HashStable` implementation from `HashMap`.

The first commit adds the `StableCompare` trait which is a companion trait to `StableOrd`. Some types like `Symbol` can be compared in a cross-session stable way, but their `Ord` implementation is not stable. In such cases, a `StableCompare` implementation can be provided to offer a lightweight way for stable sorting. The more heavyweight option is to sort via `ToStableHashKey`, but then sorting needs to have access to a stable hashing context and `ToStableHashKey` can also be expensive as in the case of `Symbol` where it has to allocate a `String`.

The rest of the commits are rather mechanical and don't overlap, so they are best reviewed individually.

Part of [MCP 533](https://github.com/rust-lang/compiler-team/issues/533).

compiler/rustc_span/src/symbol.rs

@@ -4,7 +4,9 @@
 
 use rustc_arena::DroplessArena;
 use rustc_data_structures::fx::FxIndexSet;
-use rustc_data_structures::stable_hasher::{HashStable, StableHasher, ToStableHashKey};
+use rustc_data_structures::stable_hasher::{
+    HashStable, StableCompare, StableHasher, ToStableHashKey,
+};
 use rustc_data_structures::sync::Lock;
 use rustc_macros::HashStable_Generic;
 use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
@@ -2104,6 +2106,14 @@ impl<CTX> ToStableHashKey<CTX> for Symbol {
     }
 }
 
+impl StableCompare for Symbol {
+    const CAN_USE_UNSTABLE_SORT: bool = true;
+
+    fn stable_cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.as_str().cmp(other.as_str())
+    }
+}
+
 pub(crate) struct Interner(Lock<InternerInner>);
 
 // The `&'static str`s in this type actually point into the arena.