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

[Nicolas van Kempen via cfe-commits]

================
@@ -266,3 +266,37 @@ void test(std::string s, std::string_view sv, sub_string ss, sub_sub_string sss,
   s.compare(0, 1, "ab") == 0;
   s.rfind(suffix, 1) == s.size() - suffix.size();
 }
+
+void test_substr() {
+    std::string str("hello world");
+    std::string prefix = "hello";
+    std::string_view suffix = "world";
+    
+    // Basic pattern
+    str.substr(0, 5) == "hello";
+    // CHECK-MESSAGES: :[[@LINE-1]]:{{[0-9]+}}: warning: use starts_with instead of substr() == [modernize-use-starts-ends-with]
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nicovank wrote:

```suggestion
    // CHECK-MESSAGES: :[[@LINE-1]]:{{[0-9]+}}: warning: use starts_with instead of substr
```

Here and below, this is sufficient. I'm planning to remove the `() == ` part of the diagnostic in #116132 anyway.

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