[clang-tools-extra] Adding an initial version of the "Initialized Class Members" checker. (PR #65189)

[Gábor Horváth via cfe-commits]

================
@@ -0,0 +1,56 @@
+// RUN: %check_clang_tidy %s google-cpp-init-class-members %t
+
+class PositiveDefaultedDefaultConstructor {
+public:
+  PositiveDefaultedDefaultConstructor() = default;
+  // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: constructor should initialize these fields: X
+
+private:
+  int X;
+};
+
+class PositiveDefaultedDefaultConstructorWithInitializedField {
+public:
+  PositiveDefaultedDefaultConstructorWithInitializedField() = default;
+  // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: constructor should initialize these fields: X
+
+private:
+  int X;
+  int Y = 4; // no-warning
+};
+
+class Helper {
+ public:
+  Helper(int x) : X(x) {}
+
+ private:
+  int X;
+};
+
+class PositiveDefaultedConstructorObjectAndPrimitive {
+ public:
+  PositiveDefaultedConstructorObjectAndPrimitive() = default;
+  // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: constructor should initialize these fields: Y
+
+  Helper* GetHelper() { return &X; }
+
+  void SetY(bool enabled) { Y = enabled; }
+
+  bool IsY() { return Y; }
+
+ private:
+  Helper X;
+  bool Y;
+};
+
+struct PositiveStruct {
+  // CHECK-MESSAGES: :[[@LINE-1]]:8: warning: these fields should be initialized: X, Y
+  int X;
+  int Y;
+};
+
+struct PositiveStructWithInitializedField {
+  // CHECK-MESSAGES: :[[@LINE-1]]:8: warning: these fields should be initialized: Y
+  int X = 3; // no-warning
+  int Y;
+};
----------------
Xazax-hun wrote:

I suggest you to add some TODO messages to show which other cases are you planning to cover. 
Some ideas:
* Inheritance
* Struct with anonymous members
* Struct with anonymous union members
* Unnamed structs
* Nested structs

Sometimes you might need to make a judgement call in this check.

For example, what are the plans to handle cases like:
```
struct A {
  struct {
    int a;
    int b;
  } unnamed{};
};
```

https://github.com/llvm/llvm-project/pull/65189