LLVM Dialect for pliron

This crate provides the following functionality:

1. Dialect definitions of LLVM ops, types and attributes.
2. A wrapper around llvm-sys converting to and from our LLVM dialect. This necessitates that LLVM be installed locally.

We currently support LLVM-20, and hence LLVM-20 needs to be on your computer. On Ubuntu, this means, you require the libllvm20 and libpolly-20-dev packages.

pliron-llvm also provides an llvm-opt tool.

Note: Implementation of the LLVM dialect is in-progress, not yet complete.