CI: Windows: Build LLVM and llvmpipe

CI: Windows: Build LLVM and llvmpipe

We will eventually need to build our own LLVM on Windows in order to
build libclc and other bits which are required for the d3d12 build, as
well as to be able to test SPIR-V/OpenCL on llvmpipe.

Start doing this now, building into the base container, and exercise
this by building llvmpipe under Windows.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4946>