[clang] [clang][ExprConst] Fix crash on uninitialized array subobject (PR #67817)

[Timm Baeder via cfe-commits]

tbaederr wrote:

Unfortunate, but the changes look good to me. Since the crash was introduced in clang 17, this should also be backported.

https://github.com/llvm/llvm-project/pull/67817