Placing a run-once test inside the operator lookup function caused problems with the thread sanitizer. See D122975.
D123158 attempted to address the warning with an atomic once-only approach, but missed the libcxxabi use.
This break out the operator table into a member variable, and move the test to the unit test machinery.
I have verify the unit test triggers, if you do misorder the table.
(The table could be shared across instantiations of the demangler, but that requires either inline-vars (a c++17 thing), or a separate object file.)