Add regression test for #18566
Add optional messages to the unreachable macro.

Closes #18842.
Set exceeding bitshifts lint to deny

Discussed in #18587
test: Rename files, fail -> panic.
auto merge of #18862 : murarth/rust/issue-18566-test, r=alexcrichton
auto merge of #18867 : michaelsproul/rust/unreachable-formatting, r=pcwalton

Closes #18842.
auto merge of #18872 : hirschenberger/rust/master, r=thestinger

Discussed in #18587
auto merge of #18877 : michaelsproul/rust/panic-tests, r=alexcrichton

I'm fairly sure all these tests refer to task panics. I skim read them all, but there's a small chance I renamed something too eagerly.