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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I wonder if instead of this,w e should have `InitListExpr` have some sort of `zero-filled` (ala-array-filler) section?  it seems the sparse-ness is a bit of a requirement here, and it seems silly that we'd have an implementation limit of "you tried to give us 3 items!" 

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