add debuginfo for unboxed closures

This catches two unhandled patterns in middle/trans/debuginfo.rs where a
ty::ty_unboxed_closure could show up, preventing various potential ICEs
when #[feature(unboxed_closures)] is used with `-g`.

It also updates the test case from #17093 in response to new information
from #17021 showing that the test still caused an ICE when compiling
with debuginfo.

Closes #17021.