[clang] [Clang] fix crash when constexpr evaluation encounters uninitialized GCC vector (PR #180293)

[Mariya Podchishchaeva via cfe-commits]

https://github.com/Fznamznon commented:

Thanks! I was also looking at this one, so I think the direction is correct.

https://github.com/llvm/llvm-project/pull/180293