Auto merge of #116773 - dtolnay:validatestable, r=compiler-errors

Validate `feature` and `since` values inside `#[stable(…)]`

Previously the string passed to `#[unstable(feature = "...")]` would be validated as an identifier, but not `#[stable(feature = "...")]`. In the standard library there were `stable` attributes containing the empty string, and kebab-case string, neither of which should be allowed.

Pre-existing validation of `unstable`:

```rust
// src/lib.rs

#![allow(internal_features)]
#![feature(staged_api)]
#![unstable(feature = "kebab-case", issue = "none")]

#[unstable(feature = "kebab-case", issue = "none")]
pub struct Struct;
```

```console
error[E0546]: 'feature' is not an identifier
 --> src/lib.rs:5:1
  |
5 | #![unstable(feature = "kebab-case", issue = "none")]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

For an `unstable` attribute, the need for an identifier is obvious because the downstream code needs to write a `#![feature(...)]` attribute containing that identifier. `#![feature(kebab-case)]` is not valid syntax and `#![feature(kebab_case)]` would not work if that is not the name of the feature.

Having a valid identifier even in `stable` is less essential but still useful because it allows for informative diagnostic about the stabilization of a feature. Compare:

```rust
// src/lib.rs

#![allow(internal_features)]
#![feature(staged_api)]
#![stable(feature = "kebab-case", since = "1.0.0")]

#[stable(feature = "kebab-case", since = "1.0.0")]
pub struct Struct;
```

```rust
// src/main.rs

#![feature(kebab_case)]

use repro::Struct;

fn main() {}
```

```console
error[E0635]: unknown feature `kebab_case`
 --> src/main.rs:3:12
  |
3 | #![feature(kebab_case)]
  |            ^^^^^^^^^^
```

vs the situation if we correctly use `feature = "snake_case"` and `#![feature(snake_case)]`, as enforced by this PR:

```console
warning: the feature `snake_case` has been stable since 1.0.0 and no longer requires an attribute to enable
 --> src/main.rs:3:12
  |
3 | #![feature(snake_case)]
  |            ^^^^^^^^^^
  |
  = note: `#[warn(stable_features)]` on by default
```

tests/ui/missing_const_for_fn/auxiliary/helper.rs

@@ -1,8 +1,8 @@
 // This file provides a const function that is unstably const forever.
 
 #![feature(staged_api)]
-#![stable(feature = "1", since = "1.0.0")]
+#![stable(feature = "clippytest", since = "1.0.0")]
 
-#[stable(feature = "1", since = "1.0.0")]
+#[stable(feature = "clippytest", since = "1.0.0")]
 #[rustc_const_unstable(feature = "foo", issue = "none")]
 pub const fn unstably_const_fn() {}