[clang-tools-extra] [clang-tidy] Add new check bugprone-tagged-union-member-count (PR #89925)

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+.. title:: clang-tidy - bugprone-tagged-union-member-count
+
+bugprone-tagged-union-member-count
+==================================
+
+Gives warnings for tagged unions, where the number of tags is
+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.
+
+Example
+-------
+
+.. code-block:: c++
+
+  enum tags {
+    tag1,
+    tag2,
+  };
+
+  struct taggedUnion { // warning: Tagged union has more data members (3) than tags (2)! [bugprone-tagged-union-member-count]
----------------
whisperity wrote:

```suggestion
  struct taggedUnion { // warning: Tagged union has more data members (3) than tags (2)
```

https://github.com/llvm/llvm-project/pull/89925