[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-aarch64-quick` running on `linaro-clang-aarch64-quick` while building `llvm` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/65/builds/13399

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

```
Step 5 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'LLVM-Unit :: ADT/./ADTTests/67/207' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/unittests/ADT/./ADTTests-LLVM-Unit-1562986-67-207.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=207 GTEST_SHARD_INDEX=67 /home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/unittests/ADT/./ADTTests
--

Script:
--
/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/unittests/ADT/./ADTTests --gtest_filter=EquivalenceClassesTest.MembersIterator
--
../llvm/llvm/unittests/ADT/EquivalenceClassesTest.cpp:80: Failure
Expected equality of these values:
  EC.members(EC.end()).begin()
    Which is: 8-byte object <10-02 AC-C8 FF-FF 00-00>
  EC.member_end()
    Which is: 8-byte object <00-00 00-00 00-00 00-00>


../llvm/llvm/unittests/ADT/EquivalenceClassesTest.cpp:80
Expected equality of these values:
  EC.members(EC.end()).begin()
    Which is: 8-byte object <10-02 AC-C8 FF-FF 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