[clang-tools-extra] [clang-tidy] Add modernize-substr-to-starts-with check (PR #116033)

[Nicolas van Kempen via cfe-commits]

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nicovank wrote:

FWIW I think it's a good idea in modernize-use-starts-ends-with documentation to use a table as in [readability-container-contains](https://clang.llvm.org/extra/clang-tidy/checks/readability/container-contains.html).

Here is a table with the current examples:

```
==================================================== ===========================
Expression                                           Result
---------------------------------------------------- ---------------------------
``u.find(v) == 0``                                   ``u.starts_with(v)``
``u.rfind(v, 0) != 0``                               ``!u.starts_with(v)``
``u.compare(0, v.size(), v) == 0``                   ``u.starts_with(v)``
``u.compare(u.size() - v.size(), v.size(), v) == 0`` ``u.ends_with(v)``
``u.rfind(v) == u.size() - 6``                       ``u.ends_with(v)``
==================================================== ===========================
```


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