Fix C3SXGate to_matrix method (backport #12742)

[mergify]

Summary

This PR adds the with_controlled_gate_array decorator missing in C3SXGate that enables the to_matrix method. This bug was found while porting C3SXGate to Rust (#12659).

Details and comments

I have also noticed that the path of C3SXGate is incorrect. It's currently in qiskit/circuit/library/standard_gates/x.py while it should be in qiskit/circuit/library/standard_gates/sx.py. I did not address this issue in the current PR because I believe the move to the correct location is a breaking change and should probably be left for 2.0 (although most users won't import gates directly from the file).

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This is an automatic backport of pull request #12742 done by Mergify.

[mergify]

Cherry-pick of d86f995 has failed:

On branch mergify/bp/stable/1.1/pr-12742
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  (fix conflicts and run "git cherry-pick --continue")
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  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   qiskit/circuit/library/standard_gates/x.py
	new file:   releasenotes/notes/fix-c3sx-gate-matrix-050cf9f9ac3b2b82.yaml
	modified:   test/python/circuit/test_controlled_gate.py

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   test/python/circuit/test_rust_equivalence.py

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[coveralls]

Pull Request Test Coverage Report for Build 9856351473

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 7 unchanged lines in 3 files lost coverage.
• Overall coverage remained the same at 89.942%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/expr.rs	1	94.02%
qiskit/transpiler/passes/synthesis/unitary_synthesis.py	2	88.2%
crates/qasm2/src/lex.rs	4	92.11%

Totals
Change from base Build 9839420877:	0.0%
Covered Lines:	61845
Relevant Lines:	68761

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💛 - Coveralls