📦️ Enable MLIR by default

[denialhaag]

Description

Fixes #1132

Checklist:

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
• I have added migration instructions to the upgrade guide (if needed).
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

[codecov]

Codecov Report

❌ Patch coverage is 53.84615% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../MQTOpt/Transforms/ToQuantumComputationPattern.cpp	0.0%	2 Missing ⚠️
...ct/MQTOpt/Transforms/Transpilation/LayeredUnit.cpp	33.3%	2 Missing ⚠️
...ct/MQTOpt/Transforms/MergeRotationGatesPattern.cpp	0.0%	1 Missing ⚠️
src/qir/runtime/QIR.cpp	50.0%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[burgholzer]

Okay, short summary of the latest findings here:

• Compiling with gcc under macOS seems impossible with the current setup as there is an inherent incompatibility between the LLVM build that used AppleClang and the gcc build (they do use different C++ stdlib implementations by default, so I suppose this makes sense). Either we remove gcc builds entirely on macOS and add this to the documentation somewhere, or we try to provide additional LLVM builds on macOS compiled with gcc. I'd have a slight tendency to simply remove them.
• the macOS-14 clang failures are apparently due to the clang version on macos-14 runners being too old to properly support C++20, in particular lambdas with structured bindings. We could work around this by rewriting the respective parts of the code or we could remove the runners from the matrix and declare the compiler version as not supported. The reason the MLIR tests worked here was simply that the MLIR tests where running on macos-15 with a newer version of AppleClang.
• the Windows release builds fail in the QIR runner tests. This seems fixable and is probably due to some path logic.
• the Windows debug builds all fail because of the mixture of LLVM libraries compiled in release mode and the project being compiled in debug mode. This triggers several errors because the respective C++ runtimes are not ABI compatible. For these builds to succeed, we need debug builds of LLVM, which I am trying to add in ✨🚸 Add Debug builds of the LLVM toolchain for Windows and optimize distribution munich-quantum-software/portable-mlir-toolchain#4

The rest seems to work just fine already 🙌🏼

[burgholzer]

Okay, given how we already merged the dialect redesign PR, which should not be part of v3.4, I am also going to merge this PR as soon as the CI is green in order to not have it sit around for too long and become stale again.
I'll try to get the stable v3.x branch set up afterwards as soon as possible.