[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.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291380
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