Value-init fields of WinRT structs (fix warning C24695 in generated code)

[DefaultRyan]

Current codegen was triggering Warning C24695 - Variable 'foo' is uninitialized.

This adds a default initializer to struct fields.

Before:

namespace winrt::Windows::UI::Xaml::Interop
{
    struct TypeName
    {
        hstring Name;
        winrt::Windows::UI::Xaml::Interop::TypeKind Kind;
    };
}

After:

namespace winrt::Windows::UI::Xaml::Interop
{
    struct TypeName
    {
        hstring Name {};
        winrt::Windows::UI::Xaml::Interop::TypeKind Kind {};
    };
}

Since consume* methods that return a struct are already value-initializing the return value, this has a negligible effect on the compiler codegen there. So that's nice.