Rename the ConstS::val field as kind.

And likewise for the `Const::val` method.

Because its type is called `ConstKind`. Also `val` is a confusing name
because `ConstKind` is an enum with seven variants, one of which is
called `Value`. Also, this gives consistency with `TyS` and `PredicateS`
which have `kind` fields.

The commit also renames a few `Const` variables from `val` to `c`, to
avoid confusion with the `ConstKind::Value` variant.

compiler/rustc_const_eval/src/const_eval/mod.rs

@@ -197,7 +197,7 @@ pub(crate) fn deref_const<'tcx>(
         },
     };
 
-    tcx.mk_const(ty::ConstS { val: ty::ConstKind::Value(op_to_const(&ecx, &mplace.into())), ty })
+    tcx.mk_const(ty::ConstS { kind: ty::ConstKind::Value(op_to_const(&ecx, &mplace.into())), ty })
 }
 
 #[instrument(skip(tcx), level = "debug")]