swift234

Make the functions generating LLVM IR declarations of runtime entry points available outside of IRGen.

This functionality could be re-used by e.g. LLVMPasses, which currently create LLVM IR declarations of runtime entry points on their own.

To make the function re-usable, slightly change the API of the function:
- use llvm::Module instead of IRGenModule.
- use llvm::ArrayRef instead of std::initializer_list, which allows the clients of this API to dynamically form the lists of return types and arguments.