Unfortunately there's no upstream frontend for Metal but since the id's
are now assigned by the DWARF standard I think it makes sense to have
the enums upstream to enable tools like llvm-dwarfdump. This patch
therefore uses an AArch64 test with artificially modified debug info to
verify that the Metal language id can be used.
https://dwarfstd.org/issues/241111.1.html
5d9aabbab3