[Python] Ensure attribute_to_var only accesses value when legal.

[mikeurbach]

This codepath assumes when the input attribute is not an Attribute, it has a value member. This is the case for things like StringAttr, but not for ArrayAttr. This popped up because upstream MLIR has been working to return downcasted concrete Attribute subclasses in more situations, so APIs that previously returned an Attribute might now return an ArrayAttr. A simpler testcase also reveals the issue. I've updated this path to also ensure that a value member exists before using it, and otherwise fall back to the generic code path.

[mikeurbach]

@teqdruid what do you think? It looks like this path was added to avoid going into the chain of try/except probing, but this would never have worked for passing an ArrayAttr directly to attribute_to_var, since ArrayAttr has no value. Now that more APIs return ArrayAttr directly, we're exposing this issue.

[teqdruid]

Out of curiousity, what does ArrayAttr have instead of value? I would expect it to have some member which returns an iterable of attributes.

[mikeurbach]

It defines __iter__, which returns an iterator over the attributes: https://github.com/llvm/llvm-project/blob/05abcc579244b68162b847a6780d27b22bd58f74/mlir/lib/Bindings/Python/IRAttributes.cpp#L324.

You can do things like for attr in myarray.