Rollup merge of #118029 - saethlin:allocid-gc, r=RalfJung

Expand Miri's BorTag GC to a Provenance GC

As suggested in https://github.com/rust-lang/miri/issues/3080#issuecomment-1732505573

We previously solved memory growth issues associated with the Stacked Borrows and Tree Borrows runtimes with a GC. But of course we also have state accumulation associated with whole allocations elsewhere in the interpreter, and this PR starts tackling those.

To do this, we expand the visitor for the GC so that it can visit a BorTag or an AllocId. Instead of collecting all live AllocIds into a single HashSet, we just collect from the Machine itself then go through an accessor `InterpCx::is_alloc_live` which checks a number of allocation data structures in the core interpreter. This avoids the overhead of all the inserts that collecting their keys would require.

r? ``@RalfJung``

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -107,6 +107,14 @@ impl<K: Hash + Eq, V> interpret::AllocMap<K, V> for FxIndexMap<K, V> {
         FxIndexMap::contains_key(self, k)
     }
 
+    #[inline(always)]
+    fn contains_key_ref<Q: ?Sized + Hash + Eq>(&self, k: &Q) -> bool
+    where
+        K: Borrow<Q>,
+    {
+        FxIndexMap::contains_key(self, k)
+    }
+
     #[inline(always)]
     fn insert(&mut self, k: K, v: V) -> Option<V> {
         FxIndexMap::insert(self, k, v)