👷 Lower reusable functions with standard MLIR passes

[burgholzer]

This marks the end of the first stack or PRs to add multiple function support.
Fixes #1659

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Description

Use standard MLIR passes to lower reusable functions for compiler targets. Program entry points are public, defined func.func operations, which gives symbol liveness an explicit root. Target compilation inlines reusable calls before decomposition, mapping, synthesis, and conformance checks. Target-selected QIR compilation avoids redundant early inlining.

QC export and explicit QC/QCO cleanup remove unreachable private functions. The OpenQASM and Qiskit exporters emit only gates reachable from the entry point, so unused gate declarations are dropped. Explicit cleanup also uses MLIR dead-value removal to prune unused unitary parameters and qubits together with the corresponding call operands.

No custom interprocedural framework or WireIterator state is added. qco.call already exposes positional wire correspondence through UnitaryOpInterface; generic func.call remains an intentional wire boundary.

The prerequisite structured-export changes in #2338, #2339, #2340, and #2342 are merged. This branch is rebased onto current main and requires no additional dependency.

Final-head local validation passed: release configuration and build; all 3,918 registered CTest cases with one expected skip; all 294 Qiskit translation tests; C++ lint; and repository lint. Hosted CI for the rebased commit is still running.

Codex implemented and tested the changes with independent MLIR/compiler specialist review.

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[codecov]

Codecov Report

❌ Patch coverage is 88.57143% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../Dialect/QC/Translation/TranslateQCToOpenQASM3.cpp	81.8%	4 Missing ⚠️

📢 Thoughts on this report? Let us know!

[burgholzer]

This now also looks decent and like a workable solution for now.
Custom gates will simply be inlined now as part of the target compilation. This is definitely an aspect to improve in the future.