Auto merge of #137563 - FractalFir:dep_graph_cap, r=<try>

[Perf experiment] Changed TaskDeps to start with preallocated with 1024 capacity

This is a tiny change that makes `TaskDeps::read_set` start preallocated with capacity for 1024 elements(4096 bytes).

Somewhat annoyingly, `HashSet::with_capacity` requires the hasher to implement `RandomState`. Since `FxHash` does not implement `RandomState`, I had to use `HashSet::with_capacity_and_hasher`, which required re-exporting `FxBuildHasher` in `rustc_data_structures`.

From local profiling, it looks like `TaskDeps::read_set`  is one of the most-often resized hash-sets in `rustc`.

Local perf runs indicate this is a small perf improvement(without any regressions). There is also no significant RSS increase(the RSS changes are noisy and cancel themselves out almost entirely).

Still, since the local and CI results can differ, I'll do a CI perf run before this PR can be reviewed.

`@bors` try `@rust-timer` queue

Author: bors

compiler/rustc_query_system/src/dep_graph/graph.rs

@@ -1299,12 +1299,11 @@ impl Default for TaskDeps {
             #[cfg(debug_assertions)]
             node: None,
             reads: EdgesVec::new(),
-            read_set: FxHashSet::default(),
+            read_set: FxHashSet::with_capacity_and_hasher(128, Default::default()),
             phantom_data: PhantomData,
         }
     }
 }
-
 // A data structure that stores Option<DepNodeColor> values as a contiguous
 // array, using one u32 per entry.
 struct DepNodeColorMap {