Replace LLVMBuild.txt and llvm-build script by LLVMBuild.cmake and a cmake script.
As a side effect, prune now unused parts of llvm-build, like the ability to generate Makefile Fragment.
It also removes LLVMBuild.txt for binaries (and the Tool componenet type) as they were not used anymore.
This removes a Python dependency for the build system.
Even if already covered in LLVMBuild.rst, could you document which arguments are accepted here as well?