[clang] [Serialization] Stop demote var definition as declaration (#172430) (#177117) (PR #184287)

[Aiden Grossman via cfe-commits]

https://github.com/boomanaiden154 approved this pull request.

LGTM given there seems to be appropriate test coverage for the case we found in `clang/test/Modules/module-init-forcelly-loaded-module.cpp`. Would be good if one of the clang maintainers could approve as well before landing.

https://github.com/llvm/llvm-project/pull/184287