Attributes may gain features or added flexibility over time. This patch aims to add a simple and uniform way to directly add/query for arbitrary changes in attributes, instead of having to rely on other information (e.g. version numbers, the existence of other attributes added at around the same time as the feature you're interested in, ...).
The only user of this at the moment will be https://reviews.llvm.org/D32332, so I won't tag people for review here until that lands.
Better words than "enhancement" are welcome; I tried things like __has_attribute_extension and __has_attribute_feature, but we also have both __has_extension and __has_feature, so... :)