rustc_span: Reorder some `ExpnData` fields in accordance with comments
rustc_span: Reorder some `ExpnData` fields in accordance with comments

A drive-by change.

aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust