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

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-nvptx-nvidia-win` running on `as-builder-8` while building `llvm` at step 7 "test-build-unified-tree-check-llvm".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/54/builds/7105

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 7 (test-build-unified-tree-check-llvm) failure: test (failure)
******************** TEST 'LLVM-Unit :: ADT/./ADTTests.exe/64/104' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:C:\buildbot\as-builder-8\llvm-nvptx-nvidia-win\build\unittests\ADT\.\ADTTests.exe-LLVM-Unit-1464-64-104.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=104 GTEST_SHARD_INDEX=64 C:\buildbot\as-builder-8\llvm-nvptx-nvidia-win\build\unittests\ADT\.\ADTTests.exe
--

Script:
--
C:\buildbot\as-builder-8\llvm-nvptx-nvidia-win\build\unittests\ADT\.\ADTTests.exe --gtest_filter=EquivalenceClassesTest.MembersIterator
--
C:\buildbot\as-builder-8\llvm-nvptx-nvidia-win\llvm-project\llvm\unittests\ADT\EquivalenceClassesTest.cpp(80): error: Expected equality of these values:
  EC.members(EC.end()).begin()
    Which is: 8-byte object <50-9C C2-E4 DD-01 00-00>
  EC.member_end()
    Which is: 8-byte object <00-00 00-00 00-00 00-00>


C:\buildbot\as-builder-8\llvm-nvptx-nvidia-win\llvm-project\llvm\unittests\ADT\EquivalenceClassesTest.cpp:80
Expected equality of these values:
  EC.members(EC.end()).begin()
    Which is: 8-byte object <50-9C C2-E4 DD-01 00-00>
  EC.member_end()
    Which is: 8-byte object <00-00 00-00 00-00 00-00>



********************


```

</details>

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