[ORC-RT] Make the ORC runtime C API public.

This is a first step towards allowing programs to pre-link against the ORC
runtime, which would allow us to move some code that is currently in the LLVM
OrcTarget library into the ORC runtime instead.

The C API header has limited utility as-is, but serves as a minimal first step
and provides clients with tools for interacting with wrapper functions.

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D127324