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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 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.

Fair. Sent https://discourse.llvm.org/t/rfc-clang-not-assuming-there-is-at-most-one-definition-in-a-redeclaration-chain/89360

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