[clang] [Serialization] Handle uninitialized type constraints (PR #110496)

[Younan Zhang via cfe-commits]

zyn0217 wrote:

> To summarize: Land this PR and backport it. Revert on trunk. Submit new PR for the regression test? (We want to keep that, right?)

That's just my idea, and I'd like Corentin to say something about it :)

https://github.com/llvm/llvm-project/pull/110496