[XCOFF][NFC] make StorageMappingClass/SymbolType member optional This patch makes StorageMappingClass/SymbolType member optional in class MCSectionXCOFF. Non-csect sections like debug sections have no such properties. Reviewed By: hubert.reinterpretcast Differential Revision: https://reviews.llvm.org/D96641 GitOrigin-RevId: 4c23707a418add9c584af5247faa5a9527632a0c