Some minor corrections and improvements to LangRef

Some productions in the LangRef were using undefined terminals and non-terminals, which have been added to the EBNF.
The dialect type and dialect attribute productions matched precisely the same structure and have been deduplicated.
The production for ssa-id was ambiguous but the fix is trivial (merging the leading '%') and has been applied.

Closes tensorflow/mlir#265

PiperOrigin-RevId: 282470892