[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 approved this pull request.

LGTM with the vector changed back to a SmallVector. This is a much cleaner change, thank you for spotting it!

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