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

[Florian Albrechtskirchinger via cfe-commits]

falbrechtskirchinger wrote:

@zyn0217 #111179 has fixed this crash as well. Should I strip this PR down to the unit test, or does it still make sense to you, to keep the changes?

> Will this be backported to 19.x afterwards?

I was also going to ask, though, it's now a question about #111179.

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