in nightly, std::thread_local! uses `unreachable!` where it needs to say `$crate::unreachable!`

[elidupree]

Commit 36c9045 introduced two lines that say unreachable!(), which breaks this code (playground):

#![no_implicit_prelude]
::std::thread_local! {static Q: () = (); }

I believe they need to say $crate::unreachable! instead.

Also, if there isn't one already, we should create a regression test for this - #![no_implicit_prelude] seems easy to break by accident.

[niluxv]

Fixed by #95557, now merged into master so this can be closed I think.