[demangler][NFC] OperatorInfo table unit test

Placing a run-once test inside the operator lookup function caused
problems with the thread sanitizer. See D122975.

Break out the operator table into a member variable, and move the test
to the unit test machinery.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D123390

GitOrigin-RevId: c47bcf9af6fcfd565af25ab95932d7ceadd5d5ab
1 file changed