Fix feature gate for repr(simd)

[JonathanBrouwer]

The following code currently compiles on stable rust:

#[repr(simd)]
println!()

This has the following behavior:

• 1.90 and earlier give an "unused attribute warning"
• 1.91 ..= beta give no diagnostic output
• nightly gives the following:

warning: `#[repr(simd)]` attribute cannot be used on macro calls
 --> src/main.rs:4:5
  |
4 |     #[repr(simd)]
  |     ^^^^^^^^^^^^^
  |
  = help: `#[repr(simd)]` can only be applied to structs
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: `#[warn(unused_attributes)]` (part of `#[warn(unused)]`) on by default

This PR changes this to a feature gate error.
r? @mejrs

[rustbot]

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

[mejrs]

Please amend the PR description to state that this compiles without warning on rust 1.91 ..= current beta; this warning is only shown on nightly.

View changes since this review

[JonathanBrouwer]

@rustbot ready

[mejrs]

r=me after lang decision

View changes since this review