[llvm] [ADT] use correct iterator_facade_base for SmallSetIterator (PR #180967)

[via llvm-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Linux-->
# :penguin: Linux x64 Test Results

* 169543 tests passed
* 3053 tests skipped
* 1 test failed

## Failed Tests
(click on a test name to see its output)

### LLVM-Unit
<details>
<summary>LLVM-Unit.ADT/_/ADTTests/SmallSetTest/FilterPositive</summary>

```
Script:
--
/home/gha/actions-runner/_work/llvm-project/llvm-project/build/unittests/ADT/./ADTTests --gtest_filter=SmallSetTest.FilterPositive
--
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/unittests/ADT/SmallSetTest.cpp:311
Expected equality of these values:
  PositiveIt
    Which is: 40-byte object <E0-1B B3-63 FE-7F 00-00 01-83 20-A1 54-C6 CC-86 E8-1B B3-63 FE-7F 00-00 01-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00>
  PositiveRange.end()
    Which is: 40-byte object <E8-1B B3-63 FE-7F 00-00 01-00 00-00 00-00 00-00 E8-1B B3-63 FE-7F 00-00 01-00 00-00 00-00 00-00 50-04 D6-40 00-00 00-00>


```
</details>

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the `infrastructure` label.

https://github.com/llvm/llvm-project/pull/180967