✨ Add direct OpenQASM to QC translation

[J4MMlE]

Description

This PR adds a direct QASM3 → QC dialect translation, bypassing QuantumComputation.

Previous flow:
OpenQASM -> AST -> QuantumComputation -> QC dialect
New flow:
OpenQASM -> AST -> QC dialect

The new flow reuses the existing parser and walks the AST directly to emit QC dialect ops. Skipping QuantumComputation removes an intermediate step and enables QASM3 features previously unsupported, such as gate modifiers, classical computations and control flow structures.

For now i added a --direct-import flag to invoke the new direct translation - this will eventually be dropped, so that this direct translation becomes the new default.

What works

• All standard gates plus Qiskit-style MCX variants
• Gate modifiers: ctrl @, negctrl @, inv @, nested combinations
• Register declarations, qubit/bit allocation
• Measure, reset, barrier
• if/else over quantum statements
• Hardware qubits convert to qc.static
• Broadcasting (register-width gate calls)
• User-defined compound gates (inline expansion)

Current limitations

• Gate parameters must be compile-time constants. Adding Values is a requirement to also support classical computations.
• pow modifier is unsupported (but will/should be, once it gets merged into main)
• Layout pragmas are unsupported (they have no equivalent in QC)
• classical computations and control flow unsupported (these require extension on the parser)
• if only works up to qc but should work for the whole pipeline once the scf conversion PR is in

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.

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[denialhaag]

Is there currently any significant feature that requires the legacy loader? If not, I'd say we can drop this flag and remove the legacy function already in this PR. 🤔

[burgholzer]

Yeah. There is no need for any kind of deprecation period here. Let's just use the new functionality 👍🏻

[codecov]

Codecov Report

❌ Patch coverage is 0% with 559 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../lib/Dialect/QC/Translation/TranslateQASM3ToQC.cpp	0.0%	559 Missing ⚠️

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