[clang-tools-extra] [clang-tidy] Fix bugprone-tagged-union-member-count false-positive (PR #135831)

[via cfe-commits]

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@@ -9,6 +9,9 @@ different from the number of data members inside the union.
 A struct or a class is considered to be a tagged union if it has
 exactly one union data member and exactly one enum data member and
 any number of other data members that are neither unions or enums.
+The union and enum data members that are from system header files or
+the std namespace are not considered to make up the tagged union part
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EugeneZelenko wrote:

```suggestion
the ``std`` namespace are not considered to make up the tagged union part
```

https://github.com/llvm/llvm-project/pull/135831