[clang-tools-extra] [clang-tidy] Add performance-substr-self-assignment check (PR #209657)

[via cfe-commits]

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+.. title:: clang-tidy - performance-substr-self-assignment
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+performance-substr-self-assignment
+===================================
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EugeneZelenko wrote:

```suggestion
performance-substr-self-assignment
==================================
```

https://github.com/llvm/llvm-project/pull/209657