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

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================
@@ -132,11 +157,11 @@ size_t TaggedUnionMemberCountCheck::getNumberOfValidEnumValues(
   // 3. The value of the counting enum constant is the largest out of every enum
   //    constant.
----------------
isuckatcs wrote:

So, if I understand it correctly, what we really care about is whether the value of the last enumerator equals to the number of the other enumerators.

E.g.:
```c++
enum tag_with_counter {
  tag1 = 100,
  tag2 = 200,
  tag_count = 2
};
```
In this example `tag_count` is still a counter, but point 3. doesn't apply, so the heuristics doesn't find it.

```c++
enum tag_with_counter {
  tag1 = 2,
  tag2 = 3,
  tag_count = 2
};
```
In this other example, the last enumerator is still a counter, but point 1. doesn't apply (?).

```c++
enum tag_with_counter {
  tag1,
  tag2,
  tag3
};
```
In this last example however all points apply, so the heuristics detects the last enumerator as a counter, when it's not one.

If you really want this heuristics, I suggest replacing this function with a simple check for the value of the last enumerator being equal to the number of the other enumerators. You use the name of the enumerator to determine whether it's a counter anyway.

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