Support depending on arbitrary versions of LLVM

[igaray]

Our current dependency chain looks more or less like:

stateDiagram-v2
    direction LR
    state native
    state melior
    state "mlir-sys" as mlirsys
    state "llvm-sys" as llvmsys
    state tblgen
    state "OS package manager" as package
    state LLVM
    native --> melior
    melior --> mlirsys
    melior --> llvmsys
    melior --> tblgen
    mlirsys --> package
    llvmsys --> package
    tblgen --> package
    package --> LLVM

When we require changes or fixes in LLVM we are currently forced to wait until the entire dep chain is updated. We should have a way for (at least) our CI to be able to use any version of LLVM specified by a git SHA.

We need to 1) outline a plan of steps necessary for this:

• [x] have a fork of mlir-sys under lambdaclass org
• [x] have a fork of llvm-sys under lambdaclass org
• [x] have a fork of tblgen under lambdaclass org
• [x] have a fork of melior under lambdaclass org
• [ ] script + dockerfile to build and package LLVM and upload the result as an artifact. This script and dockerfile could also be used by clients to build their deps if necessary

[edg-l]

• https://github.com/lambdaclass/mlir-sys
• https://github.com/lambdaclass/llvm-sys.rs
• https://github.com/lambdaclass/tblgen-rs
• https://github.com/lambdaclass/melior