[clang] Use the correct namespace for looking up matching operator!= (PR #68922)

[via cfe-commits]

================
@@ -64,6 +64,10 @@ C/C++ Language Potentially Breaking Changes
     ``__has_c_attribute(warn_unused_result)``,          202003, 0
     ``__has_c_attribute(gnu::warn_unused_result)``,     202003, 1
 
+- Fixed a bug in finding matching `operator!=` while adding reveresed `operator==` as
----------------
cor3ntin wrote:

```suggestion
- Fixed a bug in finding matching `operator!=` while adding reversed `operator==` as
```

https://github.com/llvm/llvm-project/pull/68922