feat:distinguish forwarddecl & empty decl

[luoliwoshang]

• forward decl with nil Field field

	forwardDecl := &ast.RecordType{
		Tag:    ast.Struct,
		Fields: nil,
	}

• empty decl

	emptyDecl := &ast.RecordType{
		Tag:    ast.Struct,
		Fields: &ast.FieldList{},
	}

---

define addition libclang wrap function will cause fail luoliwoshang#7
so we need copy the goplus/lib/clang to current project , to temporary resolve this situation,and in the futrue , libclang's llgo binding will move to llpkg.
move at #529

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.20%. Comparing base (7abb9e8) to head (1f3d3a5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #527   +/-   ##
=======================================
  Coverage   84.20%   84.20%           
=======================================
  Files          27       27           
  Lines        2747     2747           
=======================================
  Hits         2313     2313           
  Misses        391      391           
  Partials       43       43           

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