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

[Max Manolov via cfe-commits]

maxmanolov wrote:

Thanks guys, this is really helpful. On why this is scoped to designated initializers: per @efriedma-quic's breakdown, `int a[N];` and `int b[N] = {0};` cost Clang nothing because they're bss or trailing zeros, but `int c[] = {[N-1] = 0};` costs 8 bytes per skipped element in `InitListExpr` before codegen runs. And that's the OOM from #205472 so the three cases don't behave the same right now, and a general array-size flag wouldn't fix this without a separate check at the same spot. The current check only fires in `CheckDesignatedInitializer` when an array designator's end index reaches the limit, so dense brace lists and `#embed` aren't affected. Of course, I'm happy to rename if a name like `-fmax-implicit-init-list-elements` makes that clearer. And the default could be much higher than 1M since the cost is only a pointer per element. I see three options:

1. Keep the narrow flag with @AaronBallman's two diagnostics
2. Drop the flag and land only a hard ceiling near the allocation limit as a pure crash fix
3. Park this and prototype a sparse or filler representation for leading zeros, which is @erichkeane's idea

Which would you prefer?

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