Rename C-like enum to Field-less enum

There is no need to reference the C programming language to explain this concept.

Author: notriddle

src/test/parse-fail/tag-variant-disr-non-nullary.rs

@@ -10,7 +10,7 @@
 
 // compile-flags: -Z parse-only
 
-//error-pattern: discriminator values can only be used with a c-like enum
+//error-pattern: discriminator values can only be used with a field-less enum
 
 enum color {
     red = 0xff0000,