Rollup merge of #121840 - oli-obk:freeze, r=dtolnay

Expose the Freeze trait again (unstably) and forbid implementing it manually

non-emoji version of rust-lang/rust#121501

cc #60715

This trait is useful for generic constants (associated consts of generic traits). See the test (`tests/ui/associated-consts/freeze.rs`) added in this PR for a usage example. The builtin `Freeze` trait is the only way to do it, users cannot work around this issue.

It's also a useful trait for building some very specific abstrations, as shown by the usage by the `zerocopy` crate: google/zerocopy#941

cc ```@RalfJung```

T-lang signed off on reexposing this unstably: rust-lang/rust#121501 (comment)