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

[via cfe-commits]

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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
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isuckatcs wrote:

The matcher only matches `struct` IIRC.

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