contracts: added lang items that act as hooks for rustc-injected code to invoke.

see test for an example of the kind of injected code that is anticipated here.

compiler/rustc_span/src/symbol.rs

@@ -676,6 +676,7 @@ symbols! {
         const_ty_placeholder: "<const_ty>",
         constant,
         constructor,
+        contract_build_check_ensures,
         contract_check_ensures,
         contract_check_requires,
         contract_checks,