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_codegen_cranelift/src/base.rs

@@ -710,7 +710,7 @@ fn codegen_stmt<'tcx>(
                     let times = fx
                         .monomorphize(times)
                         .eval(fx.tcx, ParamEnv::reveal_all())
-                        .val()
+                        .kind()
                         .try_to_bits(fx.tcx.data_layout.pointer_size)
                         .unwrap();
                     if operand.layout().size.bytes() == 0 {