Locally pub but inaccessible items w/o stability attributes can cause an ICE

[pnkfelix]

Spawned off of #38689

There are still some cases where referencing an inaccessible item can cause the compiler to ICE.

Here is one crafted example (playpen)

fn main() {
    let a = std::sys::imp::process::process_common::StdioPipes { ..panic!() };
}

but if std's module structure is revised then that example may fail to compile without ICE'ing for other reasons, so take care in understanding the issue: the change introduced by #38689 is skipping the requirement for stability attributes is slightly too narrow, in that it uses a local notion of pub/non-pub instead of overall accessibility.

One simple solution to this may be to just follow the suggestion here #38689 (comment) and just remove the assertion entirely. (I did not want to adopt that solution for a PR to be backported to beta, but I think it is worth considering doing in nightly alone to resolve this low priority issue.)

[petrochenkov]

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error: internal compiler error: src/librustc/middle/stability.rs:547: encountered unmarked API