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

[Max Manolov via cfe-commits]

maxmanolov wrote:

Hi all, just checking in on the preferred direction here. Should I continue with the defensive diagnostic approach in this PR, or would you prefer that I investigate a compact zero-filled representation for sparse designated initializers instead?

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