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

[Jackson Stogel via cfe-commits]

jtstogel wrote:

Hi @ChuanqiXu9, thanks for addressing the repro. I've run some tests internally. This version fixes a large swath of the build errors, but there are still a good number of similar failures with `error: static assertion expression is not an integral constant expression`. Reducing the build to a smaller reproducer now, but it will unfortunately take some time (days) as failing builds consist of many thousands of files.

https://github.com/llvm/llvm-project/pull/177117