[clang] Revert "[Serialization] Stop demote var definition as declaration" (PR #176441)

[Jordan Rupprecht via cfe-commits]

rupprecht wrote:

In the interest of getting things back to green, I'd like to land this now. Precommit CI is green. It also sounds like a breakage is not surprising since this is a long standing invariant in Clang.

The repro will still be provided later, but modules failures usually take longer to reduce, so it might be a while.

https://github.com/llvm/llvm-project/pull/176441