Replace mk_foo calls with infer_foo where possible.

There are several `mk_foo`/`intern_foo` pairs, where the former takes an
iterator and the latter takes a slice. (This naming convention is bad,
but that's a fix for another PR.)

This commit changes several `mk_foo` occurrences into `intern_foo`,
avoiding the need for some `.iter()`/`.into_iter()` calls. Affected
cases:
- mk_type_list
- mk_tup
- mk_substs
- mk_const_list

compiler/rustc_middle/src/ty/context.rs

@@ -1190,7 +1190,7 @@ impl<'tcx> TyCtxt<'tcx> {
         self.mk_imm_ref(
             self.lifetimes.re_static,
             self.type_of(self.require_lang_item(LangItem::PanicLocation, None))
-                .subst(self, self.mk_substs([self.lifetimes.re_static.into()].iter())),
+                .subst(self, self.intern_substs(&[self.lifetimes.re_static.into()])),
         )
     }