[clang] [clang][Sema] Avoid out-of-memory crash on huge designated initializer indices (PR #205503)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

@ojhunt we'd usually have flags for that (like we have constexpr step limits) - I agree that hard coding a value is not great

https://github.com/llvm/llvm-project/pull/205503