Fixed parser to not try to get code gen details of function forward declarations.

src/Parser/Parser.cpp

@@ -1582,7 +1582,8 @@ void Parser::WalkFunction(clang::FunctionDecl* FD, CppSharp::AST::Function^ F,
         ParamStartLoc = VD->getLocEnd();
     }
 
-    bool CheckCodeGenInfo = !FD->isDependentContext() && !FD->isInvalidDecl();
+    bool CheckCodeGenInfo = !FD->isDependentContext() && !FD->isInvalidDecl()
+        && FD->hasBody();
 
     if (auto RD = FD->getResultType()->getAsCXXRecordDecl())
             CheckCodeGenInfo &= RD->hasDefinition();