[llvm] [ADT] Speed up FoldingSet with Algorithm R (PR #218188)

[Fangrui Song via llvm-commits]

MaskRay wrote:

I am testing whether we really need the `used` bit array as I did for DenseMap. For FoldingSet we can use the StringMap layout (still testing https://github.com/llvm/llvm-project/pull/218190)

https://llvm-compile-time-tracker.com/compare.php?from=ada22991220fe96130d7d5d69f16cb17b53e3e27&to=a696955cd3af626c92dd7cb60846cca174ee82ec&stat=instructions:u

https://github.com/llvm/llvm-project/pull/218188