Deal with llvm-project submodule

[aleasims]

Rust has a submodule llvm-project, which is a fork of original LLVM. It can be built with rustbuild and used by rustc. However since we only use rslang with circifier (which is also a LLVM fork), we probably don't need this submodule at all.

Replacing this submodule to a circifier may look sound, but since circifier is not a standalone repo we will have hard times compiling it.

The suggestion is to fully remove this submodule. If one would like to build rustc from source he then should use umbrella repo.

The only problem about removing this submodule that I see now, is that rustbuild may attempt to build LLVM and expects it to be at this path. However again: building from sources should be done from umbrella repo, where building LLVM is configured correctly.

Rust patches to LLVM

The other point here is should we bring Rust patches from llvm-project into circifier. llvm-project is not tightened to LLVM version, but to Rust version. Having another one source of updates may make updating circifier a real pain.

Rust build system automatically detects whether there are Rust to LLVM patches or not, depending on provided llvm-config path (see llvm-has-rust-patches config option). So basically Rust must work fine without these patches.