minimize header dependencies

Organize code to minimize header dependencies and improve compilation times. Use forward declarations instead of includes when possible, avoid unnecessary dependencies, and follow proper header organization patterns.

Key practices:

• Use forward declarations in headers when you only need to declare pointers or references to a type
• Implementation files (.cpp) should contain the actual includes, while headers (.h) should minimize dependencies
• Avoid including heavy headers when lighter alternatives exist
• Never use static declarations in header files as they create separate instances in each translation unit
• Organize includes to separate interface declarations from implementation dependencies

Example:

// Good: Use forward declaration in header
// MyClass.h
class WarnUnusedResultAttr; // Forward declaration
class MyClass {
  WarnUnusedResultAttr* attr;
};

// MyClass.cpp - actual include in implementation
#include "clang/AST/Attr.h"

// Bad: Heavy include in header
// MyClass.h  
#include "clang/AST/Attr.h" // Unnecessary dependency

This approach reduces compilation times, minimizes rebuild cascades when headers change, and creates cleaner module boundaries.