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

[liamappelbe]

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.