[llvm][DebugInfo] Emit DW_LNAME_Assembly for DWARFv6 assembly CUs (#183897)
Use the new DWARFv6 language code for assembly CUs. While we have a
`DW_LANG_Assembly` pre-DWARFv6 I kept the `DW_LANG_Mips_Assembler`
pre-DWARFv6 for now (I added a FIXME if we want to address it).