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

[Matthew Voss via llvm-commits]

ormris wrote:

@artagnon Hi, as the llvm-ci bot notes above, a test in this PR is failing. We would like to revert, if this will take some time to fix. We're also seeing this failure in our internal CI.

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