Auto merge of #36252 - joshtriplett:union-field-never-used, r=sanxiyn

Fix "field is never used" warning to take unions into account

When compiling code containing a union with an unused field, rustc says
"struct field is never used".

Rather than saying "struct or union", or adding logic to determine the
type of the item, just change the message to "field is never used",
dropping the "struct".

Update tests accordingly.

src/librustc/middle/dead.rs

@@ -548,7 +548,7 @@ impl<'a, 'tcx, 'v> Visitor<'v> for DeadVisitor<'a, 'tcx> {
     fn visit_struct_field(&mut self, field: &hir::StructField) {
         if self.should_warn_about_field(&field) {
             self.warn_dead_code(field.id, field.span,
-                                field.name, "struct field");
+                                field.name, "field");
         }
 
         intravisit::walk_struct_field(self, field);