refactor constant evaluation error reporting

Refactor constant evaluation to use a single error reporting function
that reports a type-error-like message.

Also, unify all error codes with the "constant evaluation error" message
to just E0080, and similarly for a few other duplicate codes. The old
situation was a total mess, and now that we have *something* we can
further iterate on the UX.

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

@@ -20,9 +20,10 @@ use std::{u8, u16, u32, u64, usize};
 
 const A_I8_T
     : [u32; (i8::MAX as i8 + 1u8) as usize]
-    //~^ ERROR mismatched types:
-    //~| expected `i8`,
-    //~| found `u8` [E0250]
+    //~^ ERROR constant evaluation error [E0080]
+    //~| mismatched types
+    //~| expected `i8`
+    //~| found `u8`
     = [0; (i8::MAX as usize) + 1];
 
 
@@ -33,7 +34,8 @@ const A_CHAR_USIZE
 
 const A_BAD_CHAR_USIZE
     : [u32; 5i8 as char as usize]
-    //~^ ERROR only `u8` can be cast as `char`, not `i8`
+    //~^ ERROR constant evaluation error
+    //~| only `u8` can be cast as `char`, not `i8`
     = [0; 5];
 
 fn main() {}