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

[Younan Zhang via cfe-commits]

zyn0217 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 

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

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