Consider other polyfills for builtins on D3D12

[cwfitzgerald]

Stemmed from #8332.

We currently need to use root constants to push 3 different values to d3d12.

• @builtin(base_vertex) needed for @builtin(vertex_index)
• @builtin(base_instance) needed for @builtin(instance_index)
• @builtin(num_workgroups)

All of these could be managed if we can somehow bind the indirect buffer to the shader so that the shader can read directly from its indirect buffer instead of using root constants and the shader polyfill. This would likely require us have multiple internal pipelines, one for direct and one for indirect. Thankfully switching between the two likely isn't amazingly common so this is probably a reasonable tradeoff. We also don't need to go through translating the shader multiple times, which is a real problem.