Cleanup: Use rustc's same_types for our same_tys

This delegates our `same_tys` to [ty::TyS::same_type][same_type] to
remove some duplication.

Our `same_tys` was introduced 4 years ago in rust-lang#730.

Before, `same_tys` was building an inference context to be able to call
`can_eq` to compare the types. The `rustc-dev-guide` has some more details
about `can_eq` in [Type Inference -> Trying equality][try_eq].

Now, using the rustc function, we are essentially comparing the `DefId`s
of the given types, which also makes more sense, IMO.

I also confirmed that the FIXME is resolved via a bit of `dbg!`, however
no UI tests were affected.

[same_type]: https://github.com/rust-lang/rust/blob/659951c4a0d7450e43f61c61c0e87d0ceae17087/src/librustc_middle/ty/util.rs#L777
[try_eq]: https://rustc-dev-guide.rust-lang.org/type-inference.html#trying-equality

clippy_lints/src/utils/mod.rs

@@ -890,17 +890,8 @@ pub fn return_ty<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, fn_item: hir::HirId) -> T
 }
 
 /// Checks if two types are the same.
-///
-/// This discards any lifetime annotations, too.
-//
-// FIXME: this works correctly for lifetimes bounds (`for <'a> Foo<'a>` ==
-// `for <'b> Foo<'b>`, but not for type parameters).
 pub fn same_tys<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
-    let a = cx.tcx.erase_late_bound_regions(&Binder::bind(a));
-    let b = cx.tcx.erase_late_bound_regions(&Binder::bind(b));
-    cx.tcx
-        .infer_ctxt()
-        .enter(|infcx| infcx.can_eq(cx.param_env, a, b).is_ok())
+    ty::TyS::same_type(a, b)
 }
 
 /// Returns `true` if the given type is an `unsafe` function.