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

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

This reverts parts of commit ac638c1f5fca36484506415319ab254ad522a692.

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 ac638c1f5fc, the next commit will reintroduce the actually intended
changes.