[clang] [Clang] Ensure the instantiation of array initializers for decltype/_typeof operator (PR #133575)

[via cfe-commits]

cor3ntin wrote:

I find myself wanting a `RequireCompleteType` (or just `CompletesType` - maybe with asserts that check that the resulting types is indeed complete) overload that does not take a diagnostic. `getCompletedType` does not make it clear what it does. And comments would help

https://github.com/llvm/llvm-project/pull/133575