Rollup merge of #66883 - eddyb:we-cant-have-nice-things, r=oli-obk

rustc_typeck: gate AnonConst's generics on feature(const_generics).

This PR employs the fix for #43408 when `#![feature(const_generics)]` is enabled, making the feature-gate the opt-in for all the possible breakage this may incur.

For example, if this PR lands, this will cause a cycle error (due to #60471):
```rust
#![feature(const_generics)]

fn foo<T: Into<[u8; 4]>>() {}
```
And so will anything with type-level const expressions, in its bounds.
Surprisingly, `impl`s don't seem to be affected (if they were, even libcore wouldn't compile).

One thing I'm worried about is not knowing how much unstable code out there, using const-generics, will be broken. But types like `Foo<{N+1}>` never really worked, and do after this PR, just not in bounds - so ironically, it's type-level const expressions that don't depend on generics, which will break (in bounds).

Also, if we do this, we'll have effectively blocked stabilization of const generics on #60471.

r? @oli-obk cc @varkor @yodaldevoid @nikomatsakis

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