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

[Oliver Hunt via cfe-commits]

ojhunt wrote:

@AaronBallman what is our usual approach to this kind of nonsense? (in the "behold I have made what is essentially a gigantic allocation at compiler time" nonsense, not that the report/fix is) Arbitrary constants seems not great, but also what other options does the clang and/or llvm infrastructure have to handle this?

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