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

[Nicolas van Kempen via cfe-commits]

================
@@ -7,26 +7,39 @@ Checks for common roundabout ways to express ``starts_with`` and ``ends_with``
 and suggests replacing with the simpler method when it is available. Notably, 
 this will work with ``std::string`` and ``std::string_view``.
 
-.. code-block:: c++
+The check handles the follow  ing expressions:
 
-  std::string s = "...";
-  if (s.find("prefix") == 0) { /* do something */ }
-  if (s.rfind("prefix", 0) == 0) { /* do something */ }
-  if (s.compare(0, strlen("prefix"), "prefix") == 0) { /* do something */ }
-  if (s.compare(s.size() - strlen("suffix"), strlen("suffix"), "suffix") == 0) {
-    /* do something */
-  }
-  if (s.rfind("suffix") == (s.length() - 6)) {
-    /* do something */
-  }
-
-becomes
+====================================================== =====================
+Expression                                              Replacement
+------------------------------------------------------ ---------------------
+``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.substr(0, v.size()) == v``                         ``u.starts_with(v)``
+``v == u.substr(0, v.size())``                         ``u.starts_with(v)``
+``u.substr(0, v.size()) != v``                         ``!u.starts_with(v)``
+``u.compare(u.size() - v.size(), v.size(), v) == 0``   ``u.ends_with(v)``
+``u.rfind(v) == u.size() - v.size()``                  ``u.ends_with(v)``
+====================================================== =====================
----------------
nicovank wrote:

```suggestion
==================================================== =====================
Expression                                           Replacement
---------------------------------------------------- ---------------------
``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.substr(0, v.size()) == v``                       ``u.starts_with(v)``
``v != u.substr(0, v.size())``                       ``!u.starts_with(v)``
``u.compare(u.size() - v.size(), v.size(), v) == 0`` ``u.ends_with(v)``
``u.rfind(v) == u.size() - v.size()``                ``u.ends_with(v)``
==================================================== =====================
```

Nit on whitespace just for consistency, and consolidate two cases into one.

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