[Modules] Do not rewrite existing decls when deserializing class fields

Classes can have implicit members that were added before fields were
deserialized. These members were previously silently removed from
`decls()` when fields were deserialized after them.

This was the root cause of a compilation error exposed in
bc95f273, added a test for it.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D148515