[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 `clang-cmake-x86_64-avx512-linux` running on `avx512-intel64` while building `llvm` at step 7 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/133/builds/12458

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

```
Step 7 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'LLVM-Unit :: ADT/./ADTTests/64/104' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:/localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/stage1/unittests/ADT/./ADTTests-LLVM-Unit-3264903-64-104.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=104 GTEST_SHARD_INDEX=64 /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/stage1/unittests/ADT/./ADTTests
--

Script:
--
/localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/stage1/unittests/ADT/./ADTTests --gtest_filter=EquivalenceClassesTest.MembersIterator
--
/localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/llvm/llvm/unittests/ADT/EquivalenceClassesTest.cpp:80: Failure
Expected equality of these values:
  EC.members(EC.end()).begin()
    Which is: 8-byte object <18-1B DF-D0 FF-7F 00-00>
  EC.member_end()
    Which is: 8-byte object <00-00 00-00 00-00 00-00>


/localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/llvm/llvm/unittests/ADT/EquivalenceClassesTest.cpp:80
Expected equality of these values:
  EC.members(EC.end()).begin()
    Which is: 8-byte object <18-1B DF-D0 FF-7F 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