Build LLVM for mingw as part of CI docker image
We really should be testing LLVM with mingw in CI. For now we can use the image linked here, but in the long term it would be better to build a version as part of the creation of the CI pipelines.
The biggest concern here is that compiling LLVM (even just the X86 backend) takes a long time, so we need to ensure that it's done in the earliest stage possible so that changes to the CI image doesn't result in recompiling all of LLVM.