[modules] Add an assert that we properly manage the IsCompleteDefinition flag
on CXXRecordDecls when merging definitions, and make it pass by not trying to
save and restore this flag across AST serialization/deserialization. For
CXXRecordDecls, we have a separate mechanism to manage this.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216633 91177308-0d34-0410-b5e6-96231b3b80d8