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

[via llvm-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Windows-->
# :window: Windows x64 Test Results

* 130785 tests passed
* 2929 tests skipped
* 1 test failed

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

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

```
Script:
--
C:\_work\llvm-project\llvm-project\build\unittests\ADT\.\ADTTests.exe --gtest_filter=SmallSetTest.FilterPositive
--
C:\_work\llvm-project\llvm-project\llvm\unittests\ADT\SmallSetTest.cpp:311
Expected equality of these values:
  PositiveIt
    Which is: 40-byte object <48-E6 98-10 40-00 00-00 01-43 97-15 ED-01 00-00 50-E6 98-10 40-00 00-00 01-00 00-00 00-00 00-00 65-00 00-00 00-00 00-00>
  PositiveRange.end()
    Which is: 40-byte object <50-E6 98-10 40-00 00-00 01-00 00-00 00-00 00-00 50-E6 98-10 40-00 00-00 01-6D DB-B6 6D-DB B6-6D C0-54 67-15 ED-01 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