[mlir] Improve BitEnumAttr, update documentation

- Add new operators to BitEnumAttr, mainly not (which only inverts
bits that can be valid bits for the attribute) and xor
- Add new bit enum utility functions: bitEnumClear(bits, bit) and
bitEnumSet(bits, bit, value=true) as they've come up in code I've been
writing that makes use of such enums
- Add rudimentary tests for the enum generator
- Update the OpDefinition documentation to make it contain a correct
example and to have it account for the changes mentioned above.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D133374