add new tuning debugger type DBX for XCOFF DWARF.
Based on this debugger type, for now, we plan to:
1: use inline string by default for XCOFF DWARF
2: generate no column info for debug line table.
3: may need to add a new DWARF flag/option, like strict DWARF to make sure DWARF info is generated under the correct DWARF version. See https://reviews.llvm.org/D99250
Nit: It would be nice to reformat all these comments for Doxygen:
DBX ///< Tune debug info for dbx.