rustc: Restrict the scope of a borrow on def_map

This addresses the ICE from #13763, but it does not allow the test to compile,
due to #13768. An alternate test was checked in in the meantime.

Closes #13763
auto merge of #13769 : alexcrichton/rust/restrict-some-scopes, r=huonw

This addresses the ICE from #13763, but it does not allow the test to compile,
due to #13768. An alternate test was checked in in the meantime.

Closes #13763