Fix LLVM 22 FFI pointer resolution for forward-declared types

On LLVM 22 (no ElaboratedType), type.pointee.canonical.declaration
resolves to the forward declaration cursor instead of the definition
cursor. forward_declaration? returns true for types like B and C that
have both a forward declaration AND a definition, incorrectly treating
them as opaque. opaque_declaration? correctly distinguishes: it only
returns true when there's no definition at all (like Opaque).

Author: cfis

lib/ruby-bindgen/generators/ffi/ffi.rb

@@ -435,7 +435,7 @@ def pointer_to_forward_declaration?(type)
         decl = type.pointee.canonical.declaration
         return false if [:cursor_invalid_file, :cursor_no_decl_found].include?(decl.kind)
 
-        decl.forward_declaration?
+        decl.opaque_declaration?
       end
 
       def figure_ffi_type(type, context = nil)