This is a follow-up for D75609. As @dblaikie suggested, it prints
the actual number for an unknown section identifier when dumping
unit index sections.
Differential Revision: https://reviews.llvm.org/D75668
A DWARFSectionKind is read from input. It is not validated on parsing,
so an unexpected value may result in reaching llvm_unreachable() in
DWARFUnitIndex::getColumnHeader() when dumping the index section.
Differential Revision: https://reviews.llvm.org/D75609