[mlir][ods] Allow sharding of op definitions

Adds an option to `mlir-tblgen -gen-op-defs` `op-shard-count=N` that divides the
op class definitions and op list into N segments, e.g.

```
// mlir-tblgen -gen-op-defs -op-shard-count=2

void FooDialect::initialize() {
  addOperations<
  >();
  addOperations<
  >();
}

```

When split across multiple source files, this can help significantly improve
dialect compile time for dialects with a large opset.

stack-info: PR: https://github.com/llvm/llvm-project/pull/89423, branch: users/mogball/pr_1