Serialization: Emit the final diagnostic state last, almost NFC

Emit the final diagnostic state last to match source order.  This also
prepares for a follow-up commit for implicit modules.

There's no real functionaliy change, just a slightly different AST file
format.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@300024 91177308-0d34-0410-b5e6-96231b3b80d8