Recursive const stability checks do not apply in const items

All stable const fn in core/alloc/std are checked to ensure that they, transitively, do not use any unstable const features.

However, the same is not the case for const items. A stable fn can use array lengths, and a stable trait can have associated const, that use arbitrary unstable const features. That seems bad? I'm not entirely sure how to fix that though -- these items don't themselves have any stability, after all. They are just use in a large function (that may not even be a const fn).

One example where we are actually doing this is here:
https://github.com/rust-lang/rust/blob/7f74c894b0e31f370b5321d94f2ca2830e1d30fd/library/std/src/sys/pal/unix/process/process_unix.rs#L813
CMSG_SPACE is a const fn defined in libc. It is not subject to any recursive const stability checks.

Cc @rust-lang/wg-const-eval @compiler-errors

Start by investigating the recursive const stability checks and the const item use in library/std/src/sys/pal/unix/process/process_unix.rs around CMSG_SPACE. Determine how const items and associated consts should be checked, then verify that unstable const features are rejected transitively in these cases.