Reject impl const Trait since the right syntax is const impl Trait now

[oli-obk]

I also let some smaller rustfmting of ui tests through, as rustfmt immediately formats impl const Trait to const impl Trait. This is also the reason I expect that this change will break very little nightly users, so we don't even need to add some helpful diagnostic.

r? @fee1-dead

[rustbot]

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @lcnr

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

rustfmt is developed in its own repository. If possible, consider making this change to rust-lang/rustfmt instead.

cc @rust-lang/rustfmt

stdarch is developed in its own repository. If possible, consider making this change to rust-lang/stdarch instead.

cc @Amanieu, @folkertdev, @sayantn

[rustbot]

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