Rollup merge of #120958 - ShoyuVanilla:remove-subst, r=oli-obk

Dejargonize `subst`

In favor of #110793, replace almost every occurence of `subst` and `substitution` from rustc codes, but they still remains in subtrees under `src/tools/` like clippy and test codes (I'd like to replace them after this)

compiler/rustc_mir_transform/src/instsimplify.rs

@@ -208,7 +208,7 @@ impl<'tcx> InstSimplifyContext<'tcx, '_> {
         // Only bother looking more if it's easy to know what we're calling
         let Some((fn_def_id, fn_args)) = func.const_fn_def() else { return };
 
-        // Clone needs one subst, so we can cheaply rule out other stuff
+        // Clone needs one arg, so we can cheaply rule out other stuff
         if fn_args.len() != 1 {
             return;
         }