[clang-tools-extra] [clang-tidy] Create bugprone-public-enable-shared-from-this check (PR #102299)

[Julian Schmidt via cfe-commits]

================
@@ -0,0 +1,34 @@
+.. title:: clang-tidy - bugprone-incorrect-enable-shared-from-this
+
+bugprone-incorrect-enable-shared-from-this
+=======================================
+
+Checks if class/struct publicly derives from ``std::enable_shared_from_this``,
+because otherwise when ``shared_from_this`` is called it will throw 
+``std::bad_weak_ptr``.
+
+Consider the following code:
+
+.. code-block:: c++
+    #include <memory>
+
+    class BadExample : std::enable_shared_from_this<BadExample> {
+    // warning: inheritance from std::enable_shared_from_this 
+    // should be public inheritance,
+    // otherwise the internal weak_ptr won't be initialized 
+    // [bugprone-incorrect-enable-shared-from-this]
----------------
5chmidti wrote:

The diagnostics of a check are normally not copy-pasted to the documentation. Maybe: 
> privately inheriting from `std::enable_shared_from_this` results in an uninitialized `weak_ptr` when calling `shared_from_this`

Or add smaller comments:

```
 #include <memory>

    // private inheritance
    class BadExample : std::enable_shared_from_this<BadExample> {
        public:

        // `shared_from_this` returns uninitialized `weak_ptr`
        BadExample* foo() { return shared_from_this().get(); }
        void bar() { return; }
    };

    void using_not_public() {
        auto bad_example = std::make_shared<BadExample>();
        auto* b_ex = bad_example->foo();
        b_ex->bar();
    }
```

https://github.com/llvm/llvm-project/pull/102299