Reading from a ConstantBuffer within a template fails compilation in a non-library profile

[sndels]

Reading from a ConstantBuffer within a template causes error: External function used in non-library profile: : \01??$Fn@N@@YANN@Z. Making the read within a separate function seems to be a workaround. This could be tied to #3731, but the error is quite different.

Minimal example, reproduces with dndxc.exe from de6a8ed's AppVeyor build
Fails: -enable-templates -DFAILURE_CASE
Compiles: -enable-templates

struct ConstantsStruct
{
    float Val : Val;
};
ConstantBuffer<ConstantsStruct> Constants;

#ifdef FAILURE_CASE
template<typename T>
T Fn(T input)
{
    T ret = input - Constants.Val;

    return ret;
}

#else

float ReadConstant()
{
    return Constants.Val;
}
template<typename T>
T Fn(T input)
{
    T ret = input - ReadConstant();

    return ret;
}

#endif

void main()
{
    Fn(1.f);
}

Edit: Made example more minimal

[llvm-beanz]

To throw some context in here as breadcrumbs. When you dump the AST for the failure case, the template instantiation is incomplete. The cause of this is a failure in the tree transformation copying the AST from the template into the instantiation. The failure occurs because the tree transformation tries to convert the member access following C++ casting rules, which really only support casting along inheritance lines for member access. Because this member access is a ConstantBuffer, we need to support FlatConversion casting when transforming the AST.