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

[Florian Albrechtskirchinger via cfe-commits]

falbrechtskirchinger wrote:

> I think the best past forward would be to close this PR, and to open a new one targeting the 10.x branch, which a clear explanation that it's not needed in trunk. would that work for you @falbrechtskirchinger ?

Will do.

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