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

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https://github.com/whisperity requested changes to this pull request.

I found a very convoluted and somewhat silly edge case where I was able to defeat the matchers to make them produce a match in a position where it is clearly not intended. So the matchers will need to be given some thought, my idea is to have them constricted in a way to report only into non-implicit code, but perhaps this needs some more elaboration.

Lambdas implicitly compile down to an unnamed `CXXRecordDecl`, and if you have captures in a lambda, they become (unnamed) fields of this class.

https://godbolt.org/z/rvfY5K18T

```cpp
enum E { A };
union U {
    int A;
};

int main() {
    enum E e;
    union U u;
    auto L = [e, u] () {};
}
```

```
Match #1:

<source>:9:14: note: "root" binds here
    9 |     auto L = [e, u] () {};
      |              ^
<source>:9:15: note: "tags" binds here
    9 |     auto L = [e, u] () {};
      |               ^
<source>:9:18: note: "union" binds here
    9 |     auto L = [e, u] () {};
      |                  ^
```

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