⚗️ MLIR - Investigate loosening exact LLVM version pin for external plugins

[flowerthrower]

Currently, the Catalyst Plugin CI requires installing LLVM from source (either through Catalyst from source or directly LLVM from source). This is not ideal as it takes forever. We should investigate at some point whether there is anything we can do to make this work with pre-compiled LLVM versions.

I'd still like to investigate at some point whether there is anything we can do to make this work with pre-compiled LLVM versions. But this will have to wait for a future PR. It would be great to create a tracking issue for this though.

Originally posted by @burgholzer in munich-quantum-toolkit/core#881 (comment)

[burgholzer]

The above description actually has two components:

• We currently need to pin to the exact LLVM commit catalyst is using to make the plugin work
• As a consequence of the above, we need to build the LLVM and MLIR components from source.

Currently, we also build these from source for the Windows builds in our CI, which has already caused some pain when waiting for CI.

Two things to do here:

• try and loosen up the dependency on the exact version (i.e., commit) of LLVM
• set up a system that allows us to not have to rebuild MLIR/LLVM all over every time and that provides pre-compiled packages for several versions. One concrete proposal for this would be a GitHub action similar to https://github.com/cda-tum/setup-z3

This issue should rather focus on the former, while I will create a separate issue for the latter.

[burgholzer]

Created munich-quantum-toolkit/core#1124 for tracking the second issue.