Forbid generic parameters in the type of const generic parameters

[lcnr]

e.g. do not allow fn foo<T, const N: T>() and foo<const N: usize, const M: [u8; N]>().

While we may want to support this in the future, this should not be part of the MVP and causes a lot of problems
with our current approach. It causes both ICE ICE baby and breaks foo<const N: [u8; { 1+ 4 }]>() because { 1 + 4 } contains N itself in its substitutions.

What probably has to done here:

• change name resolution to forbid mentions of parameters.
• stop using the parent generics in generics_of.

cc @eddyb @varkor @yodaldevoid

[lcnr]

note: please ping me on zulip if you want to work on this.

[varkor]

I believe we already forbade generic type parameters in const generic types, but may have forgotten about other const generics. I'll review the PR soon.

[lcnr]

Afaict we removed that ban for some reason. E0671 is currently unused.