Merge OpProperty and Traits into OpTrait

They are essentially both modelling MLIR OpTrait; the former achieves the
purpose via introducing corresponding symbols in TableGen, while the latter
just uses plain strings.

Unify them to provide a single mechanism to avoid confusion and to better
reflect the definitions on MLIR C++ side.

Ideally we should be able to deduce lots of these traits automatically via
other bits of op definitions instead of manually specifying them; but not
for now though.

PiperOrigin-RevId: 232191401