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

[via cfe-commits]

bgra8 wrote:

Heads-up @ChuanqiXu9 013b345af992f66d5ecfd168844ebfc6956ccae0 is causing serious compilation time regressions (more than 70% increase in CPU time) in some of the translation units at google.

https://github.com/llvm/llvm-project/pull/172430