Auto merge of #113522 - fmease:generic-consts, r=cjgillot

Implement generic const items

This implements generic parameters and where-clauses on free and associated const items under the experimental feature gate `generic_const_items`. See rust-lang/lang-team#214.

Tracking issue: #113521.
Fixes #104400.

This PR doesn't include rustfmt support as per [nightly style procedure](https://github.com/rust-lang/style-team/blob/master/nightly-style-procedure.md) and it doesn't add rustdoc JSON support (see [related Zulip topic](https://rust-lang.zulipchat.com/#narrow/stream/266220-rustdoc/topic/Rustdoc.20JSON.3A.20Experimental.20rustc.20features)).

CC `@scottmcm` `@compiler-errors` `@WalterSmuts`
r? `@oli-obk`

<details><summary>Resolved Questions</summary>

* Q: Should `const ADD<const N: usize, const M: usize>: usize = N + M; ADD::<0, 1>` trigger the error *generic parameters may not be used in const operations* (which can be unlocked with `#![feature(generic_const_exprs)]`). Currently it doesn't. Or does this fall under [this paragraph](https://github.com/rust-lang/rust/blob/71f71a5397c42fec01f5c1045c638d961fa9f7ca/compiler/rustc_resolve/src/late.rs#L191)?
  * A: No, https://github.com/rust-lang/rust/pull/113522#issuecomment-1628634092
* Q: Should `const UNUSED: () = () where String: Copy;` (with `#![feature(trivial_bounds)]` and with `UNUSED` unused) succeed compilation? Currently it doesn't: *evaluation of constant value failed // entering unreachable code*
  * A: Yes, but postponed until stabilization (latest), https://github.com/rust-lang/rust/pull/113522#issuecomment-1628634092

</details>

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