* Generic mlir.ir.Attribute class. * First standard attribute (mlir.ir.StringAttr), following the same pattern as generic vs standard types. * NamedAttribute class. Differential Revision: https://reviews.llvm.org/D86250
* Also raises an exception on parse error. * Removes placeholder smoketest. * Adds docstrings. Differential Revision: https://reviews.llvm.org/D86046