rustc: Fix def ids of xcrate-reexported items

This was just a typo in the decoder using the source crate's number rather than
the destination crate's number of a reexport.

Closes #13872
auto merge of #13873 : alexcrichton/rust/issue-13872, r=Aatch

This was just a typo in the decoder using the source crate's number rather than
the destination crate's number of a reexport.

Closes #13872