[llvm][DebugInfo] Add support for emitting DW_AT_language_name (#162621)
Depends on:
* https://github.com/llvm/llvm-project/pull/162445
* https://github.com/llvm/llvm-project/pull/162449
Emit `DW_AT_language_name` (new in DWARFv6) if `DICompileUnit` has a
`sourceLanguageName` field. Emit a `DW_AT_language` otherwise.