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

[Alexis Engelke via llvm-commits]

aengelke wrote:

NB: sqlite is a suboptimal benchmark here, several of FoldingSet's expensive uses reside in Clang and are primarily triggered from larger C++ workloads.

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