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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> Heads-up @ChuanqiXu9 [013b345](https://github.com/llvm/llvm-project/commit/013b345af992f66d5ecfd168844ebfc6956ccae0) is causing serious compilation time regressions (more than 70% increase in CPU time) in some of the translation units at google.

Please send a reproducer. I can image it relates the change in SemaType.cpp but I feel better to have a reproducer so that I can work locally instead imaging.

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