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

[Florian Albrechtskirchinger via cfe-commits]

falbrechtskirchinger wrote:

> @falbrechtskirchinger Ooops, yes, it's true that the bug is gone w/ #111179.
> 
> But I'd say if we're going to cherry-pick #111179, we have to backport its precedent #103867 as well, so I think we can probably go ahead with this one and backport it to 19, and then we can revert this one on the trunk - this is indeed a bit awkward, but it depends whether it's worth backporting #103867 @cor3ntin

I'd like this crash to be fixed in 19.x.


> (Again, I apologize for not looking into it so carefully)

No worries.

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

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