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

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h-vetinari wrote:

> I'd like to land this in two weeks if no objections come in.

Clang 22 branches in a few days, and this should not land before that IMO. Especially given that there was no discussion (that I'm aware of) about breaking the previously valid AST invariant following:

> Maybe it's time to change that, given that modules is now a standard language feature. But I think this is probably something the Clang Area Team should consider and make a conscious decision on.





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