[llvm] [ADT] Speed up FoldingSet with linear probing and Algorithm R deletion (PR #218190)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

> @MaskRay @kazutakahirata Maybe summarize all 3 approaches on discourse? I find it difficult to keep track of 3 competing PRs...

I'll do that.  Thanks!

https://github.com/llvm/llvm-project/pull/218190