[clang] [Clang] prevent assertion failure by avoiding casts on type declarations that require complete types (PR #101426)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

This looks good but you need to add a bit more details to your summary, more specifically what case caused the bug to emerge.

I am also curious why this does not show up in C++, we obtain a similar diagnostic but no crash. 

https://github.com/llvm/llvm-project/pull/101426