Check repr attribute consistency at check time, not translation.

Note that raising an error during trans doesn't stop the compile or cause
rustc to exit with a failure status, currently, so this is of more than
cosmetic importance.

src/test/compile-fail/tag-variant-disr-dup.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-//error-pattern:discriminator value already exists
+//error-pattern:discriminant value already exists
 
 // black and white have the same discriminator value ...