[mlir][bytecode] Add builtin dialect version (#184678)
This adds a singular Builtin dialect version for use with bytecode
serialization. This version is not currently print unless set and not 0
(not planned a bump until next LLVM version). Created a unit test as
this was easiest way to track.
Additionally add emitWarning virtual method to DialectBytecodeReader,
mirroring emitError.
Tested on old mlir-opt reader, and could read, so should be non-breaking
change.
