[ORC] Add RPC and serialization support for Errors and Expecteds.

This patch allows Error and Expected types to be passed to and returned from
RPC functions.

Serializers and deserializers for custom error types (types deriving from the
ErrorInfo class template) can be registered with the SerializationTraits for
a given channel type (see registerStringError in RPCSerialization.h for an
example), allowing a given custom type to be sent/received. Unregistered types
will be serialized/deserialized as StringErrors using the custom type's log
message as the error string.

llvm-svn: 300167