Auto merge of #74877 - lcnr:min_const_generics, r=oli-obk

Implement the `min_const_generics` feature gate

Implements both https://github.com/rust-lang/lang-team/issues/37 and https://github.com/rust-lang/compiler-team/issues/332.

Adds the new feature gate `#![feature(min_const_generics)]`.
This feature gate adds the following limitations to using const generics:
- generic parameters must only be used in types if they are trivial. (either `N` or `{ N }`)
- generic parameters must be either integers, `bool` or `char`.

We do allow arbitrary expressions in associated consts though, meaning that the following is allowed,
even if `<[u8; 0] as Foo>::ASSOC` is not const evaluatable.
```rust
trait Foo {
    const ASSOC: usize;
}

impl<const N: usize> Foo for [u8; N] {
    const ASSOC: usize = 64 / N;
}
```

r? @varkor cc @eddyb @withoutboats

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