Rename variadic to c_variadic

Function signatures with the `variadic` member set are actually
C-variadic functions. Make this a little more explicit by renaming the
`variadic` boolean value, `c_variadic`.

src/librustc/ty/instance.rs

@@ -65,7 +65,7 @@ impl<'a, 'tcx> Instance<'tcx> {
                 sig.map_bound(|sig| tcx.mk_fn_sig(
                     iter::once(*env_ty.skip_binder()).chain(sig.inputs().iter().cloned()),
                     sig.output(),
-                    sig.variadic,
+                    sig.c_variadic,
                     sig.unsafety,
                     sig.abi
                 ))