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

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

non-emoji version of https://github.com/rust-lang/rust/pull/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: https://github.com/google/zerocopy/issues/941

cc ```@RalfJung```

T-lang signed off on reexposing this unstably: https://github.com/rust-lang/rust/pull/121501#issuecomment-1969827742

compiler/rustc_span/src/symbol.rs

@@ -864,6 +864,7 @@ symbols! {
         format_placeholder,
         format_unsafe_arg,
         freeze,
+        freeze_impls,
         freg,
         frem_algebraic,
         frem_fast,