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

[via cfe-commits]

higher-performance wrote:

I just realized there is another optimization we could do in `push_back` (similar motivation as what @erichkeane mentioned [here](#discussion_r1982274548), but different):

We could avoid the duplication check in `push_back`, and defer it to the `contains()`/`view()` accessors, thus making `push_back` simply do `if (!Value.getMemoizationData()) { ... }`. If we do such a thing, then we'd probably want to amortize it so that the number of unprocessed entries doesn't grow unboundedly. We could keep the number of unprocessed elements within (say) 25% of the size of the processed elements.

Thoughts on this? I'm inclined to give it a try to see if it's worth the code complexity.

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