Entry point specialisation logic lacks support for variadic generics

[daljit46]

Currently, the compilation API does not currently support specializing a shader entry point that uses variadic generics (e.g., <each T : IInterface>).
When attempting to specialize such an entry point, IEntryPoint::getSpecializationParamCount() returns 0. Any subsequent call to IEntryPoint::specialize() with type arguments fails, as the API expects zero arguments.

To reproduce this define a shader with a variadic generic entry point:

    interface ISomeInterface {}
    struct MyOperation1 : ISomeInterface {}
    struct MyOperation2 : ISomeInterface {}

    [shader("compute")]
    [numthreads(1, 1, 1)]
    void main<each Operation : ISomeInterface>()
    {
        // ...
    }

On the C++ side:

    Slang::ComPtr<slang::IComponentType> entryPoint = ... // Get the "main" entry point
    // This will return 0
    int paramCount = entryPoint->getSpecializationParamCount();

    std::array<slang::SpecializationArg, 1> specializationArgs =
    {
        {
            slang::SpecializationArg::Kind::Type,
            slangModule->getLayout()->findTypeByName("HighQuality")
        }
    };
    Slang::ComPtr<slang::IComponentType> specializedEntryPoint;
    // The following attempt to specialize will fail
    Slang::ComPtr<slang::IBlob> diagnosticsBlob;
    SlangResult result = entryPoint->specialize(specializationArgs.data(), specializationArgs.size(),  specializedEntryPoint.writeRef(), diagnosticsBlob.writeRef());

The specialize call fails with the following error:

error 38025: expected 0 specialization arguments (1 provided)

It was pointed out to me on Discord by @csyonghe that this is because the specialisation API doesn't support variadic generics yet.