[llvm] Reland [EquivClasses] Introduce members iterator-helper (PR #130319)

[Vitaly Buka via llvm-commits]

vitalybuka wrote:

> Changes: Fix the expectations in EquivalenceClassesTest.MemberIterator, also fixing a build failure.

Please include into the description the original review.

https://github.com/llvm/llvm-project/pull/130319