[ORC] Move Orc RPC code into Shared, rename some RPC types.

Moves all headers from Orc/RPC to Orc/Shared, and from the llvm::orc::rpc
namespace into llvm::orc::shared. Also renames RPCTypeName to
SerializationTypeName and Function to RPCFunction.

In addition to being a more reasonable home for this code, this will make it
easier for the upcoming Orc runtime to re-use the Serialization system for
creating and parsing wrapper-function binary blobs.