Feature Request: Distinguish Empty Struct Definition from Forward Declaration

[luoliwoshang]

Current Problem:
The current implementation in inComplete() function (type.go:336-338) incorrectly treats empty struct definitions and forward declarations as the same thing by only checking if the fields list is empty:

https://github.com/goplus/llcppg/blob/5c906680a5817e206ebf77640f05d0505a6a1aa5/cl/internal/convert/type.go#L332-L338

This causes both cases to be treated as incomplete types:

• Empty struct definition: struct A {}
• Forward declaration: struct A;
  Impact:
• Empty structs are incorrectly marked as forward declarations
• Type completeness detection is inaccurate
• May affect code generation and type checking

[luoliwoshang]

Done in #527