Fix invalid check-cfg Cargo feature diagnostic help
#118213 added specialized diagnostic for Cargo `feature` cfg. However when providing an empty `#[cfg(feature)]` condition the suggestion would suggest adding `feature` as a feature in `Cargo.toml` (wtf!).
This PR removes the invalid logic, which even brings a nice improvement.
```diff
--> $DIR/cargo-feature.rs:18:7
|
LL | #[cfg(feature)]
- | ^^^^^^^
+ | ^^^^^^^- help: specify a config value: `= "bitcode"`
|
= note: expected values for `feature` are: `bitcode`
- = help: consider defining `feature` as feature in `Cargo.toml`
```
The first commit add a test showing the bug and the second commit fixes the bug.
`@rustbot` label +F-check-cfg
e930ea274e