// If it returns the empty string, the call did not need to be rewritten.
func (p *Package) rewriteCall(f *File, call *Call) (string, bool) {
	// This is a call to C.xxx; set goname to "xxx".
	// It may have already been mangled by rewriteName.
	var goname string
	switch fun := call.Call.Fun.(type) {
	case *ast.SelectorExpr:
		goname = fun.Sel.Name
	case *ast.Ident:
		goname = strings.TrimPrefix(fun.Name, "_C2func_")