[mlir] Add method to populate default attributes

Previously default attributes were only usable by way of the ODS generated
accessors, but this was undesirable as
1. The ODS getters could construct Attribute each get request;
2. For non-C++ uses this would require either duplicating some of tee default
   attribute generating or generating additional bindings to generate methods;
3. Accessing op.getAttr("foo") and op.getFoo() would return different results;
Generate method to populate default attributes that can be used to address
these.

This merely adds this facility but does not employ by default on any path.

Differential Revision: https://reviews.llvm.org/D128962