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

[via cfe-commits]

================
@@ -0,0 +1,66 @@
+.. 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 tags2 {
+    tag1,
+    tag2,
+  };
+
+  struct taggedunion { // warning: Tagged union has more data members than tags! Data members: 3 Tags: 2 [bugprone-tagged-union-member-count]
+    enum tags2 kind;
+    union {
+      int i;
+      float f;
+      char *str;
+    } data;
+  };
+
+  enum tags4 {
+    tag1,
+    tag2,
+    tag3,
+    tag4,
+  };
+  
+  struct taggedunion { // warning: Tagged union has more fewer members than tags! Data members: 3 Tags: 4 [bugprone-tagged-union-member-count]
+    enum tags4 kind;
+    union {
+      int i;
+      float f;
+      char *str;
+    } data;
+  };
+
+Counting enum constant heuristic
+--------------------------------
+
+Some programmers use enums in such a way, where the value of the last enum 
+constant is used to keep track of how many enum constants have been declared.
+
+.. code-block:: c++
+
+  enum tags_with_counter {
+    tag1, // is 0
+    tag2, // is 1
+    tag3, // is 2
+    tags_count, // is 3
+  };
+
+The checker detects this usage pattern heuristically and does not include
----------------
EugeneZelenko wrote:

```suggestion
The check detects this usage pattern heuristically and does not include
```

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