TableGen: fix builder generation for optional attributes

The recently introduced support for generating MLIR Operations with optional
attributes did not handle the formatted string emission properly, in particular
it did not escape `{` and `}` in calls to `formatv` leading to assertions
during TableGen op definition generation.  Fix this by splitting out the
unncessary braces from the format string.  Additionally, fix the emission of
the builder argument comment to correctly indicate which attributes are indeed
optional and which are not.

PiperOrigin-RevId: 236832230