[Serialization] Don't try to complete the redeclaration chain in

ASTReader after we start writing

This is intended to mitigate
https://github.com/llvm/llvm-project/issues/61447.

Before the patch, it takes 5s to compile test.cppm in the above
reproducer. After the patch it takes 3s to compile it. Although this
patch didn't solve the problem completely, it should mitigate the
problem for sure. Noted that the behavior of the patch is consistent
with the comment of the originally empty function
ASTReader::finalizeForWriting. So the change should be consistent with
the original design.