Implement __array__ for qasm2._DefinedGate (backport #12119)

[mergify]

Summary

Gates defined from OpenQASM 2 should have a well-defined matrix form (up to global phase) whenever all the constituent parts of the definition do. This manually makes such a matrix available.

Details and comments

Fix #12100.

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This is an automatic backport of pull request #12119 done by [Mergify](https://mergify.com).

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[coveralls]

Pull Request Test Coverage Report for Build 9403194051

Details

• 5 of 6 (83.33%) changed or added relevant lines in 1 file are covered.
• 13 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.005%) to 89.629%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/qasm2/parse.py	5	6	83.33%

Files with Coverage Reduction	New Missed Lines	%
qiskit/transpiler/passes/synthesis/unitary_synthesis.py	2	88.2%
crates/qasm2/src/lex.rs	5	92.62%
crates/qasm2/src/parse.rs	6	97.61%

Totals
Change from base Build 9276092364:	-0.005%
Covered Lines:	62259
Relevant Lines:	69463

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💛 - Coveralls