rustdoc: Fixup links to enum variants

Pages aren't emitted for enum variants, so whenever we're linking to an enum
variant instead link to the parent enum instead.

Closes #12289

Author: alexcrichton

src/librustdoc/html/render.rs

@@ -565,9 +565,16 @@ impl DocFolder for Cache {
             clean::StructItem(..) | clean::EnumItem(..) |
             clean::TypedefItem(..) | clean::TraitItem(..) |
             clean::FunctionItem(..) | clean::ModuleItem(..) |
-            clean::ForeignFunctionItem(..) | clean::VariantItem(..) => {
+            clean::ForeignFunctionItem(..) => {
                 self.paths.insert(item.id, (self.stack.clone(), shortty(&item)));
             }
+            // link variants to their parent enum because pages aren't emitted
+            // for each variant
+            clean::VariantItem(..) => {
+                let mut stack = self.stack.clone();
+                stack.pop();
+                self.paths.insert(item.id, (stack, "enum"));
+            }
             _ => {}
         }