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

[via cfe-commits]

================
@@ -97,6 +97,13 @@ Improvements to clang-tidy
 New checks
 ^^^^^^^^^^
 
+- New :doc:`performance-substr-self-assignment
+  <clang-tidy/checks/performance/substr-self-assignment>` check.
+
+  Finds cases where a string variable is assigned the result of calling
+  ``substr()`` on itself, which creates an unnecessary temporary. Suggests
+  using ``erase()`` to modify the string in-place instead.
----------------
EugeneZelenko wrote:

```suggestion
  ``substr()`` on itself.
```

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