Prepare DtoCallFunction() for runtime function calls

[kinke]

By also taking DValue* args, not just Expression*s.

[kinke]

A first step towards calling runtime functions with the DtoCallFunction() machinery. What's still missing:

1. Right now, we only have a LL function for each runtime function. We'll need to keep the associated TypeFunction* around too in order to create the DValue* fnval argument for DtoCallFunction().
2. Replace all direct LL function calls by DtoCallFunction() calls, using DValue* args instead of LL args etc.

[JohanEngelen]

Nice that you are working on the follow-up to #1208 !
I was thinking about adding a map to gen/runtime.h from functionstringname -> (llvm::Function, TypeFunction).

[JohanEngelen]

Perhaps you can use llvm::SmallVector here (number of arguments is usually small I'd think).

[dnadlinger]

Could you incorporate Johan's suggestions and rebase, please?

[kinke]

I'm not a huge fan of llvm::ArrayRef due its missing support for initializer lists in LLVM 3.5 (and 3.6?) and I doubt a llvm::SmallVector provides any measurable performance benefit over a single small heap allocation (size known a priori, elements are pointers only, each DValue arg allocated on the heap as well iirc), but I can still be persuaded. ;)

I still need to figure out how to structure the logging, as the args are currently evaluated outside the DtoCallFunction logger scope.

[JohanEngelen]

Yeah, forget about my comments :)

[redstar]

Rebase on ltsmaster and merge?

[kinke]

Finally rebased incl. taking care of logging.