Qiskit and its version of `qelib1.inc` give `C3SXGate` different names:
`c3sx` and `c3sqrtx` respectively.  At this point, changing either of
these two names is likely to cause user pain, and it should not be
difficult for the OpenQASM 2 exporter to simply know about this
remapping, like it knows about other standard gate mappings.  In
practice, the structure of the exporter makes this rather difficult, but
we need not expose that complexity to users.