[clang-tools-extra] [clang-tidy] Add `readability-redundant-tag` check (PR #210007)

[via cfe-commits]

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+.. title:: clang-tidy - readability-redundant-tag
+
+readability-redundant-tag
+=========================
+
+Finds redundant uses of the ``class``, ``struct``, ``union``, and ``enum``
+keywords in C++ declarations and provides fix-it hints to remove them.
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EugeneZelenko wrote:

I don't think that `fix-it hints` should be stressed, `list.rst` entry should be enough. Same in Release Notes.

https://github.com/llvm/llvm-project/pull/210007