Fix clangsharp_Type_getTemplateArgument for record decl

[xoofx]

Hey,
While trying to upgrade CppAst to 14.0, I got into trouble with clangsharp_Type_getTemplateArgument that was not able to recover template arguments for a record/struct:

template <typename T, typename U>
struct TemplateStruct
{
    T field0;
    U field1;
};

struct Struct2
{
};

TemplateStruct<int, Struct2> exposed;

This PR is trying to fix the issue by using the same code that is in clang_Type_getNumTemplateArguments/Type.GetTemplateArguments.