[ffigen] Parse `ns_returns_retained` and `ns_consumed` for blocks

We have all the infra for handling these annotations in ffigen, but libclang doesn't seem to have a way of accessing `__attribute__((ns_returns_retained))` and `__attribute__((ns_consumed))` annotations on blocks.

For methods and top level functions, these annotations are on the `CXCursor` of the declaration, but for blocks I only have a `CXType` with no declaration cursor. If I annotate a block with `NS_RETURNS_RETAINED`, the `CXType`'s kind changes from `FunctionProto` to `Unexposed`. If I annotate it with `__attribute__((ns_returns_retained))` the kind changes to `Attributed`, which is a bit more promising, but I can't find a way of getting the attribute. If I annotate one of the args with `__attribute__((ns_consumed))`, nothing happens at all.

Start by tracing ffigen’s existing handling for ns_returns_retained and ns_consumed on methods and top-level functions. Investigate how libclang represents these annotations on block CXType values, including FunctionProto, Unexposed, and Attributed kinds; done means both block annotations can be parsed consistently with existing support.

Written by the indexing model from the issue text.