build D3D12 driver on CI
Now that the Windows CI platform is stable (after reworking both nginx and the Windows VM host networking setup), we can actually build on MSVC as part of CI. The patches in this tree add support for building libclc as part of the Windows CI base container, then add support for actually building the CLC compiler stack and the D3D12 Gallium driver in CI.
As part of this, one test is disabled; saturated f32 -> i64/u64 conversions are still broken, which should be addressed as part of #3824. However, disable it until it's fixed.