[llvm] [ADT] ImutAVLFactory: avoid overhead of vector for freelist (PR #196534)

[Christoph Cullmann via llvm-commits]

christoph-cullmann wrote:

Given the freeList will just take the left or right pointers, given the stuff we add to the freeList will have them released before anyways, then one doesn't need to add stuff back, but I could be wrong with that.

https://github.com/llvm/llvm-project/pull/196534