[clang-tools-extra] [clang-tidy] Enhance modernize-use-starts-ends-with to handle substr patterns (PR #116033)

[Nicolas van Kempen via cfe-commits]

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@@ -244,7 +244,10 @@ Changes in existing checks
 
 - Improved :doc:`modernize-use-starts-ends-with
   <clang-tidy/checks/modernize/use-starts-ends-with>` check to handle two cases
-  that can be replaced with ``ends_with``
+  that can be replaced with ``ends_with`` and detect patterns using ``substr``
+  that can be replaced with ``starts_with``. Now handles cases like
+  ``str.substr(0, n) == "literal"``, with support for length determination through
+  integer literals, ``strlen()``, and ``size()``/``length()`` member functions.
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nicovank wrote:

```suggestion
- Improved :doc:`modernize-use-starts-ends-with
  <clang-tidy/checks/modernize/use-starts-ends-with>` check to handle two new
  cases from ``rfind`` and ``compare`` to ``ends_with``, and one new case from
  ``substr`` to ``starts_with``.
```

Details are assumed / in documentation anyway.

https://github.com/llvm/llvm-project/pull/116033