Consistently use 'supertrait'.

A subset of places referred to 'super-trait', so this changes them
to all use 'supertrait'. This matches 'supertype' and some other
usages. An exception is 'auto-trait' which is consistently used
in that manner.

compiler/rustc_middle/src/ty/mod.rs

@@ -599,7 +599,7 @@ impl<'tcx> Predicate<'tcx> {
         //   where both `'x` and `'b` would have a DB index of 1.
         //   The substitution from the input trait-ref is therefore going to be
         //   `'a => 'x` (where `'x` has a DB index of 1).
-        // - The super-trait-ref is `for<'b> Bar1<'a,'b>`, where `'a` is an
+        // - The supertrait-ref is `for<'b> Bar1<'a,'b>`, where `'a` is an
         //   early-bound parameter and `'b' is a late-bound parameter with a
         //   DB index of 1.
         // - If we replace `'a` with `'x` from the input, it too will have