When generating debug info collect information on calling convention for types (http://www.dwarfstd.org/ShowIssue.php?issue=141215.1) and pass it to backend.
Be best to avoid having to 'fixup' the argabi after the fact - and instead build the type with the right argabi from the get-go.
Could you file a bug/feature request with the DWARF committee? & maybe we could/should use an implementation defined extension value here? (assuming the DW_CC set has an implementation defined space - which I really hope it does)
I'd probably make this a struct (and drop the 'public:' section since it'll be the default then) - since it doesn't seem to matter here.