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

[via cfe-commits]

github-actions[bot] wrote:

<!--LLVM CODE LINT COMMENT: clang-tidy-->


:warning: C/C++ code linter, clang-tidy found issues in your code. :warning:

<details>
<summary>
You can test this locally with the following command:
</summary>

```bash

git diff -U0 origin/main...HEAD -- clang-tools-extra/clang-tidy/performance/SubstrSelfAssignmentCheck.cpp clang-tools-extra/clang-tidy/performance/SubstrSelfAssignmentCheck.h clang-tools-extra/clang-tidy/performance/PerformanceTidyModule.cpp |
python3 clang-tools-extra/clang-tidy/tool/clang-tidy-diff.py   -path build -p1 -quiet
```

</details>

<details>
<summary>
View the output from clang-tidy here.
</summary>

```
clang-tools-extra/clang-tidy/performance/SubstrSelfAssignmentCheck.cpp:119:3: warning: variable 'Diag' of type 'DiagnosticBuilder' can be declared 'const' [misc-const-correctness]
  119 |   auto Diag = diag(AssignExpr->getOperatorLoc(),
      |   ^
      |        const
```

</details>


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