Documentation for llvmcall

[toivoh]

I've really started to enjoy llvmcall, and I think I have a reasonable idea how it works thanks to the close analogy with ccall. Still, both for myself and for the sake of others, it would be great to have some docs for it.

[StefanKarpinski]

+1. I've also wondered to what extent we should try to use llvmcall to replace intrinsics and built-ins.

[wbhart]

Currently the only examples I can find are with parameters that are bitstypes or tuples thereof. It would be great to have even a single example where jl_value_t* s are passed. Everything I've tried has failed. LLVM just responds with jl_value_t* type not known.

[tkelman]

closed by #10760