Auto merge of #74595 - lcnr:ConstEvaluatable-fut-compat, r=oli-obk

make `ConstEvaluatable` more strict

relevant zulip discussion: https://rust-lang.zulipchat.com/#narrow/stream/146212-t-compiler.2Fconst-eval/topic/.60ConstEvaluatable.60.20generic.20functions/near/204125452

Let's see how much this impacts. Depending on how this goes this should probably be a future compat warning.

Short explanation: we currently forbid anonymous constants which depend on generic types, e.g. `[0; std::mem::size_of::<T>]` currently errors.

We previously checked this by evaluating the constant and returned an error if that failed. This however allows things like
```rust
const fn foo<T>() -> usize {
    if std::mem::size_of::<*mut T>() < 8 { // size of *mut T does not depend on T
        std::mem::size_of::<T>()
    } else {
        8
    }
}

fn test<T>() {
    let _ = [0; foo::<T>()];
}
```
which is a backwards compatibility hazard. This also has worrying interactions with mir optimizations (https://github.com/rust-lang/rust/pull/74491#issuecomment-661890421) and intrinsics (#74538).

r? `@oli-obk` `@eddyb`

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