allow_internal_unstable does not work for min_specialization

[clubby789]

This also applies to proc macros
https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/.60SpecOptionPartialEq.60.20error

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=45d821a040bf825549fe07efa2c87572

#![allow(internal_features)]
#![feature(allow_internal_unstable)]

#[allow_internal_unstable(min_specialization)]
macro_rules! test {
    () => {
        struct T<U>(U);
        trait Tr {}
        impl<U> Tr for T<U> {}
        impl Tr for T<u8> {}
    }
}

test! {}

   Compiling playground v0.0.1 (/playground)
error[E0119]: conflicting implementations of trait `Tr` for type `T<u8>`
  --> src/lib.rs:10:9
   |
9  |         impl<U> Tr for T<U> {}
   |         ------------------- first implementation here
10 |         impl Tr for T<u8> {}
   |         ^^^^^^^^^^^^^^^^^ conflicting implementation for `T<u8>`
...
14 | test! {}
   | -------- in this macro invocation```

[compiler-errors]

There are probably also a bunch of other features that simply check validity with tcx.features().feature_name rather than checking for feature enablement a span. Would be cool to find out which ones do.

[Noratrieb]

I would guess mst of them, all the ones that are not used in a macro in the standard library.