Implement the DWARF 6 language and version attributes. (#89980)

This patch adds DWARF constants for DW_AT_language_name and
DW_AT_language_version to Dwarf.def and Dwarf.h.

While the DWARF 6 spec is not finalized, the constants are published on
the DWARF website and considered stable, with idea being that the list
published on dwarfstd.org is the authoritative source that is being
continuously updated between DWARF revisions, as new languages are being
developed.

https://dwarfstd.org/languages-v6.html

My main motivation for adding this is to use in
https://github.com/llvm/llvm-project/pull/89981