Handle list of const intrinsics properly for 1.49

I tried this code:

#![feature(intrinsics)]

mod intrinsics {
    extern "rust-intrinsic" {
        fn unreachable() -> !;
    }
}

#[inline]
#[stable(feature = "unreachable", since = "1.27.0")]
// #[rustc_const_unstable(feature = "const_unreachable_unchecked", issue = "53188")]
pub const unsafe fn unreachable_unchecked() -> ! {
    // SAFETY: the safety contract for `intrinsics::unreachable` must
    // be upheld by the caller.
    unsafe { intrinsics::unreachable() }
}

I expected to see this happen: no errors. This is taken from core 1.49.

Instead, this happened:

test.rs:15:14: error: only functions marked as ‘const’ are allowed to be called from constant contexts [E0015]
   15 |     unsafe { intrinsics::unreachable() }
      |              ^~~~~~~~~~

The way this is handled is in collaboration with #2999, as per the doc:

Furthermore this attribute is needed to mark an intrinsic as const fn, because there's no way to add const to functions in extern blocks for now.

https://rustc-dev-guide.rust-lang.org/stability.html#rustc_const_unstable

(and similarly for #[rustc_const_stable])

Reproduce the reported E0015 failure using the test.rs snippet in the issue. Read the rustc_const_unstable guidance and related issue #2999 to understand how const intrinsics are represented. Done means the core 1.49 example compiles without errors and the list of const intrinsics is handled correctly.