[clang] Reduce memory usage in AST parent map generation by lazily checking if nodes have been seen (PR #129934)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Checking in - has everybody had a chance to review this? Can we merge?

Was @cor3ntin correct here?

> I think we are a bit confused, so can someone tell me if I'm wrong
>
>    N is always going to be fairly small (and always 1 in the non-template case)
    We care about the order of parents
    We are just trying to avoid duplicate
    Intrusive linked list


https://github.com/llvm/llvm-project/pull/129934