Correctly process flatten fields in enum variants

- Fix incorrect deserialization of variants that doesn't contain flatten field when other contains
- Fix a panic when deriving `Deserialize` for an enum with tuple and struct with flatten field

Fixes (2):
    regression::issue2565::simple_variant
    regression::issue1904 (compilation)

serde_derive/src/internals/ast.rs

@@ -85,6 +85,7 @@ impl<'a> Container<'a> {
                     for field in &mut variant.fields {
                         if field.attrs.flatten() {
                             has_flatten = true;
+                            variant.attrs.mark_has_flatten();
                         }
                         field.attrs.rename_by_rules(
                             variant