Rollup merge of rust-lang#86189 - JohnTitor:relate-fn-pub, r=Aaron1011

Make `relate_type_and_mut` public

rust-lang#85343 improved diagnostics around `Relate` impls but made `relate_type_and_mut` private, which was accessible as `relate` previously. This makes it public so that we can use it on rust-semverver.

r? ```@Aaron1011```

Author: JohnTitor

compiler/rustc_middle/src/ty/relate.rs

@@ -112,7 +112,7 @@ pub trait Relate<'tcx>: TypeFoldable<'tcx> + Copy {
 ///////////////////////////////////////////////////////////////////////////
 // Relate impls
 
-fn relate_type_and_mut<'tcx, R: TypeRelation<'tcx>>(
+pub fn relate_type_and_mut<'tcx, R: TypeRelation<'tcx>>(
     relation: &mut R,
     a: ty::TypeAndMut<'tcx>,
     b: ty::TypeAndMut<'tcx>,