In order to meaningfully generate getters and setters from IRDL, it
makes sense to embed the names of operands, results, etc. in the IR
definition. This PR introduces this feature. Names are constrained
similarly to TableGen names.
Being able to add custom dialects is one of the big missing pieces of
the C API. This change should make it achievable via IRDL. Hopefully
this should open custom dialect definition to non-C++ users of MLIR.