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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

@AaronBallman should take a look at this as well.  We might take a look at `SetVector` and see if there are any learnings we can take from that.  It has a set and a vector together, but might have some interesting things.

Finally... I wonder if we were to just store this as a sorted-vector of `DynTypedNode` + `insert order`, then do a copy & sort on access of `view`.  

Can we do some benchmarking to figure out which things are done the 'most often' with what values of `N`?  it might better guide this.

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