Detect missing else in block with return

[estebank]

error: expected identifier, found keyword `return`
   --> /home/gh-estebank/rust/compiler/rustc_lint/src/default_could_be_derived.rs:249:13
    |
248 |         let hir::VariantData::Struct { fields, recovered: Recovered::No } = data {
    |                                                                             ---- while parsing this struct
249 |             return;
    |             ^^^^^^ expected identifier, found keyword
    |
help: escape `return` to use it as an identifier
    |
249 |             r#return;
    |             ++

This should detect that there's a missing else: = data else {.