rustc_const_eval: always demand typeck_tables for evaluating constants.

src/test/compile-fail/const-eval-overflow-4b.rs

@@ -20,8 +20,9 @@ use std::{u8, u16, u32, u64, usize};
 
 const A_I8_T
     : [u32; (i8::MAX as i8 + 1u8) as usize]
-    //~^ ERROR constant evaluation error [E0080]
+    //~^ ERROR mismatched types
     //~| expected i8, found u8
+    //~| ERROR the trait bound `i8: std::ops::Add<u8>` is not satisfied
     = [0; (i8::MAX as usize) + 1];
 
 
@@ -32,8 +33,7 @@ const A_CHAR_USIZE
 
 const A_BAD_CHAR_USIZE
     : [u32; 5i8 as char as usize]
-    //~^ ERROR constant evaluation error
-    //~| only `u8` can be cast as `char`, not `i8`
+    //~^ ERROR only `u8` can be cast as `char`, not `i8`
     = [0; 5];
 
 fn main() {}