Auto merge of #86860 - fee1-dead:stabilize, r=LeSeulArtichaut

Auto merge of #86860 - fee1-dead:stabilize, r=LeSeulArtichaut

Stabilize `arbitrary_enum_discriminant`

Closes #60553.

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## Stabilization Report

_copied from https://github.com/rust-lang/rust/issues/60553#issuecomment-865922311_

### Summary

Enables a user to specify *explicit* discriminants on arbitrary enums.

Previously, this was hard to achieve:

```rust
#[repr(u8)]
enum Foo {
    A(u8) = 0,
    B(i8) = 1,
    C(bool) = 42,
}
```

Someone would need to add 41 hidden variants in between as a workaround with implicit discriminants.

In conjunction with [RFC 2195](https://github.com/rust-lang/rfcs/blob/master/text/2195-really-tagged-unions.md), this feature would provide more flexibility for FFI and unsafe code involving enums.

### Test cases

Most tests are in [`src/test/ui/enum-discriminant`](https://github.com/rust-lang/rust/tree/master/src/test/ui/enum-discriminant), there are two [historical](https://github.com/rust-lang/rust/blob/master/src/test/ui/parser/tag-variant-disr-non-nullary.rs) [tests](https://github.com/rust-lang/rust/blob/master/src/test/ui/parser/issue-17383.rs) that are now covered by the feature (removed by this pr due to them being obsolete).

### Edge cases

The feature is well defined and does not have many edge cases.
One [edge case](https://github.com/rust-lang/rust/issues/70509) was related to another unstable feature named `repr128` and is resolved.

### Previous PRs

The [implementation PR](https://github.com/rust-lang/rust/pull/60732) added documentation to the Unstable Book, https://github.com/rust-lang/reference/pull/1055 was opened as a continuation of https://github.com/rust-lang/reference/pull/639.

### Resolution of unresolved questions

The questions are resolved in https://github.com/rust-lang/rust/issues/60553#issuecomment-511235271.

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(someone please add `needs-fcp`)