fixes for fallout in tests/compile-fail

Author: pnkfelix

src/test/compile-fail/const-eval-overflow.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(negate_unsigned)]
+
 #![allow(unused_imports)]
 
 // Note: the relevant lint pass here runs before some of the constant

src/test/compile-fail/enum-discrim-too-small.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(negate_unsigned)]
+
 #[repr(u8)] //~ NOTE discriminant type specified here
 enum Eu8 {
     Au8 = 23,

src/test/compile-fail/lint-exceeding-bitshifts.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(negate_unsigned)]
 #![deny(exceeding_bitshifts)]
 #![allow(unused_variables)]
 #![allow(dead_code)]

src/test/compile-fail/lint-type-limits.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(negate_unsigned)]
 #![allow(dead_code)]
 
 // compile-flags: -D unused-comparisons