Fix generation of native instance constructors in C++ generator.
Fix generation of native instance constructors in C++ generator.

mono/CppSharp,ddobrev/CppSharp,ddobrev/CppSharp,zillemarco/CppSharp,mono/CppSharp,zillemarco/CppSharp,zillemarco/CppSharp,mono/CppSharp,mono/CppSharp,zillemarco/CppSharp,ddobrev/CppSharp,ddobrev/CppSharp,ddobrev/CppSharp,zillemarco/CppSharp,mono/CppSharp,mono/CppSharp