Add a test for PartialEq across Allocators breaking inference (#113283)

Verify that `PartialEq` implementations do not break type inference
when comparing types across different allocators. This catches a
regression in current nightly introduced in fc04089fa0 (alloc: Allow
comparing `Box`s over different allocators")

`Box` is the only type that currently impelements this, but tests are
included for `Rc` and `Arc` to prevent future regresssions.

tests/ui/type-inference/issue-113283-alllocator-trait-eq.rs

@@ -0,0 +1,18 @@
+// run-pass
+// Verify that PartialEq implementations do not break type inference when
+// accepting types with different allocators
+
+use std::rc::Rc;
+use std::sync::Arc;
+
+
+fn main() {
+    let boxed: Vec<Box<i32>> = vec![];
+    assert_eq!(boxed, vec![]);
+
+    let rc: Vec<Rc<i32>> = vec![];
+    assert_eq!(rc, vec![]);
+
+    let arc: Vec<Arc<i32>> = vec![];
+    assert_eq!(arc, vec![]);
+}