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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Just some minor nits from me so far.

I think there may be some additional performance benefits we could eek out, but I don't think they need to be done in this patch (or done at all, it's speculative). One is that I noticed `ASTContext::getTraversalScope()` is returning a vector by copy and that doesn't seem necessary. Another is that perhaps we want to use a bloom filter here instead on the assumption that the parent map will always be fairly large. But again, this is all speculation.

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