[clang-tools-extra] [clang-tidy] Fix crash and infinite loop on incomplete types (PR #219781)

[Zeyi Xu via cfe-commits]

https://github.com/zeyi2 commented:

> should I keep the new tests in separate files like they are in this current PR, or merge them into the existing test files?

I think we should put them into the existing files.

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Also, it would be better if you can split the PR into two separate PRs, one for `misc-multiple-inheritance` and one for `readability-identifier-naming`

https://github.com/llvm/llvm-project/pull/219781