I didn't realise LLVM supported such a wide variety of calling conventions.
It would be cool if they supported OpenVMS/x86-64 calling convention–even not on OpenVMS. Why? Well, OpenVMS calling convention has this cool little feature which nobody else seems to have – the parameter count to a function is passed in a register. What this means–people always ask "how can a C variadic function know how many parameters it was passed?" And the usual answer is – you need to add an argument to explicitly pass an argument count (the computation of which can be automated using preprocessor voodoo), or you need to pass some kind of printf-style format argument, or a sentinel value (terminal NULL), or whatever. "Why can't we just have a va_count() which tells us how many arguments we were passed?" "Not possible, per the calling convention, the caller doesn't tell us."
However, for OpenVMS, there actually is such a thing as va_count() – it pulls it from that extra register (the "Argument Information Register"–which also encodes some info on their types, so you can know whether each parameter is passed in an integer register or a floating point one.) Anyway, if LLVM/Clang/etc supported OpenVMS calling convention on other platforms, they could enjoy va_count() too, you'd just have to declare that as the calling convention for your function.