Revert parts of "use derive proc macro to impl SessionDiagnostic"

This reverts parts of commit ac638c1.

During rebase, this commit accidentally reverted unrelated changes to
the subdiagnostic derive (those allowing multipart_suggestions to be
derived). This commit reverts all changes to the subdiagnostic code made
in ac638c1, the next commit will reintroduce the actually intended
changes.

Author: Xiretza