ODS: support UnitAttr in Operation definitions

A recent commit introduced UnitAttr into the ODS but did not include the
support for using UnitAttrs in operation definitions (only patterns were
supported).  Extend the ODS definition of UnitAttr to be usable in operation
definition by providing a trivial builder and an accessor that returns "true"
if the unit attribute is present since the attribute presence itself has
meaning.

Additionally, test that unit attributes are effectively rewritten in patterns
in addition to the already available FileCheck tests of the generated rewriter
code.

PiperOrigin-RevId: 259560653