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

[Mariya Podchishchaeva via cfe-commits]

https://github.com/Fznamznon commented:

Mostly looks ok to me. Given that it adds some user interface I would still be more comfortable waiting for final approval from @AaronBallman whose replies might be delayed due to WG14 meeting happening next week

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