Add nullability annotations to AST pointer types

[soutaro]

Summary

This PR adds Clang nullability annotations (_Nullable and _Nonnull) to pointer types throughout the RBS AST header and implementation files. These annotations help static analysis tools and IDEs better understand which pointers can be NULL and which cannot.

Key Changes

• Added nullability macros (RBS_NULLABLE and RBS_NONNULL) in include/rbs/defines.h that expand to Clang's _Nullable and _Nonnull attributes on Clang, and to nothing on other compilers for compatibility

• Annotated struct fields across all AST node types with appropriate nullability markers:

  • Required pointer fields marked with RBS_NONNULL
  • Optional pointer fields marked with RBS_NULLABLE
  • Examples include class declarations, method definitions, type parameters, and various annotation types

• Annotated function signatures for all AST constructor and utility functions:

  • Return types annotated (e.g., rbs_node_list_t *RBS_NONNULL)
  • Parameter types annotated (e.g., rbs_allocator_t *RBS_NONNULL)
  • Functions like rbs_hash_find() and rbs_hash_get() marked as returning RBS_NULLABLE since they can return NULL

• Updated template files (templates/template.rb and templates/include/rbs/ast.h.erb) to generate nullability annotations automatically for constructor parameters based on whether fields are optional or required

Implementation Details

• The nullability annotations are conditionally compiled using #ifdef __clang__ to ensure compatibility with non-Clang compilers
• Constructor parameters for pointer types are annotated based on the field's optional status: optional fields use RBS_NULLABLE, required fields use RBS_NONNULL
• The template system was enhanced to track nullability information for function parameters and generate appropriate annotations

https://claude.ai/code/session_013XvhPzhCAZGQ9qniMY1YQY