Edit CMake and remove unecessary dependencies

[maxgillett]

See https://github.com/tonlabs/TON-Compiler repo structure for an example of how to remove everything not required for backend compilation.

[vivekvpandya]

The linked code seems to be using old LLVM repo. So if you would like to have regular updates from LLVM community and if up streaming this work to LLVM community is desired then its not good idea to make things as per above project.
See https://llvm.org/docs/Proposals/GitHubMove.html

Correct me if this understanding is wrong.

[maxgillett]

Ah, I'm not suggesting that we change the directory structure, just that this repo should be a standalone product and only contain dependencies required to build the compiler. I'm not advocating for making any changes to the target independent code unless they're absolutely necessary. It's definitely a goal to one day upstream the backend target to the llvm-project repo.