[llvm] [ADT] Speed up FoldingSet with Swiss Table (PR #218156)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

> I am testing an alternative that uses linear probing + Algorithm R deletion.

I happen to be playing with Algorithm R also.  In the same setup for this PR, I get 1.50% improvement on a wide variety of C++ source files (in excess of 25000 files).

https://github.com/llvm/llvm-project/pull/218156