[Orc][RPC] Add an APICalls utility for grouping RPC funtions for registration.

APICalls allows groups of functions to be composed into an API that can be
registered as a unit with an RPC endpoint. Doing registration on a-whole API
basis (rather than per-function) allows missing API functions to be detected
early.

APICalls also allows Function membership to be tested at compile-time. This
allows clients to write static assertions that functions to be called are
members of registered APIs.

llvm-svn: 291380